diff --git a/src/js/CalculationModel.js b/src/js/CalculationModel.js
index f605b07..ed15826 100644
--- a/src/js/CalculationModel.js
+++ b/src/js/CalculationModel.js
@@ -187,6 +187,7 @@ export default class CalculationModel extends Model {
             this.prevNumber = this.expression.charAt(i) + this.prevNumber;
             i--;
         }
+        console.log(this.prevNumber)
     }
 
     _isDigitOrDot(char) {
diff --git a/tests/CalculationModel.test.js b/tests/CalculationModel.test.js
new file mode 100644
index 0000000..4bd1eb8
--- /dev/null
+++ b/tests/CalculationModel.test.js
@@ -0,0 +1,838 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import CalculationModel from "../src/js/CalculationModel.js";
+
+describe('Allowed characters as first input', {}, () => {
+  let model;
+  beforeEach(() => {
+    model = new CalculationModel();
+  })
+
+  it('should allow any digit', {}, () => {
+    for (let i = 0; i < 10; i++) {
+      model.selectDigit(i.toString());
+      expect(model.expression).toBe(i.toString());
+      model = new CalculationModel();
+    }
+  })
+
+  it('should allow [ ( ]', {}, () => {
+    model.selectLeftParen();
+    expect(model.expression).toBe('(');
+  })
+
+  it('should not allow [ ) ]', {}, () => {
+    model.selectRightParen();
+    expect(model.expression).toBe('');
+  })
+
+  it('should allow [ - ]', {}, () => {
+    model.selectOperation('-');
+    expect(model.expression).toBe('~');
+  })
+
+  it('should not allow [ + ]', {}, () => {
+    model.selectOperation('+');
+    expect(model.expression).toBe('');
+  })
+
+  it('should not allow [ * ]', {}, () => {
+    model.selectOperation('*');
+    expect(model.expression).toBe('');
+  })
+
+  it('should not allow [ / ]', {}, () => {
+    model.selectOperation('/');
+    expect(model.expression).toBe('');
+  })
+
+  it('should not allow [ . ]', {}, () => {
+    model.selectDot();
+    expect(model.expression).toBe('');
+  })
+
+  it('should not change expression when delete last input is selected', {}, () => {
+    model.delete();
+    expect(model.expression).toBe('');
+  })
+})
+
+describe('Input [ . ] in the middle of expression', {}, () => {
+  let model;
+  beforeEach(() => {
+    model = new CalculationModel();
+  })
+
+  it('should allow [ . ] if current number does not have [ . ]', {}, () => {
+    model.selectDigit('1');
+    model.selectDot();
+    expect(model.expression).toBe('1.');
+  })
+
+  it('should allow [ . ] if current number does not have [ . ] and there is a previous number that has [.]', {}, () => {
+    model.selectDigit('1');
+    model.selectDot();
+    model.selectDigit('2');
+    model.selectOperation('+');
+    model.selectDigit('3');
+    model.selectDot();
+    expect(model.expression).toBe('1.2+3.');
+  })
+
+  it('should not allow [ . ] if current number has [ . ]', {}, () => {
+    model.selectDigit('1');
+    model.selectDot();
+    model.selectDigit('2');
+    model.selectDot();
+    expect(model.expression).toBe('1.2');
+  })
+
+  it('should not allow [ . ] after [ ( ]', {}, () => {
+    model.selectLeftParen();
+    model.selectDot();
+    expect(model.expression).toBe('(');
+  })
+
+  it('should not allow [ . ] after [ ) ]', {}, () => {
+    model.selectLeftParen();
+    model.selectDigit('1');
+    model.selectRightParen();
+    model.selectDot();
+    expect(model.expression).toBe('(1)');
+  })
+
+  it('should not allow [ . ] after [ + ]', {}, () => {
+    model.selectDigit('1');
+    model.selectOperation('+');
+    model.selectDot();
+    expect(model.expression).toBe('1+');
+  })
+
+  it('should not allow [ . ] after [ - ]', {}, () => {
+    model.selectDigit('1');
+    model.selectOperation('-');
+    model.selectDot();
+    expect(model.expression).toBe('1-');
+  })
+
+  it('should not allow [ . ] after [ - ] (negation)', {}, () => {
+    model.selectOperation('-');
+    model.selectDot();
+    expect(model.expression).toBe('~');
+  })
+
+  it('should not allow [ . ] after [ * ]', {}, () => {
+    model.selectDigit('1');
+    model.selectOperation('*');
+    model.selectDot();
+    expect(model.expression).toBe('1*');
+  })
+
+  it('should not allow [ . ] after [ / ]', {}, () => {
+    model.selectDigit('1');
+    model.selectOperation('/');
+    model.selectDot();
+    expect(model.expression).toBe('1/');
+  })
+})
+
+describe('Input a digit in the middle of expression', {}, () => {
+  let model;
+  beforeEach(() => {
+    model = new CalculationModel();
+  })
+
+  it('should allow any digit after any digit if a number does not start with [ 0 ]', {}, () => {
+    for (let i = 0; i < 10; i++) {
+      for (let j = 0; j < 10; j++) {
+        model.selectDigit(1);
+        model.selectDigit(i.toString());
+        model.selectDigit(j.toString());
+        expect(model.expression).toBe('1' + i.toString() + j.toString());
+        model = new CalculationModel();
+      }
+    }
+  })
+
+  it('should not allow any digit as the second digit if number starts with [ 0 ]', {}, () => {
+    for (let i = 0; i < 10; i++) {
+      model.selectDigit('0');
+      model.selectDigit(i.toString());
+      expect(model.expression).toBe('0');
+      model = new CalculationModel();
+    }
+  })
+
+  it('should allow any digit after [ + ]', {}, () => {
+    for (let j = 0; j < 10; j++) {
+      model.selectDigit('1');
+      model.selectOperation('+');
+      model.selectDigit(j.toString());
+      expect(model.expression).toBe('1+' + j.toString());
+      model = new CalculationModel();
+    }
+  })
+
+  it('should allow any digit after [ - ]', {}, () => {
+    for (let j = 0; j < 10; j++) {
+      model.selectDigit('1');
+      model.selectOperation('-');
+      model.selectDigit(j.toString());
+      expect(model.expression).toBe('1-' + j.toString());
+      model = new CalculationModel();
+    }
+  })
+
+  it('should allow any digit after [ - ] (negation)', {}, () => {
+    for (let j = 0; j < 10; j++) {
+      model.selectOperation('-');
+      model.selectDigit(j.toString());
+      expect(model.expression).toBe('~' + j.toString());
+      model = new CalculationModel();
+    }
+  })
+
+  it('should allow any digit after [ * ]', {}, () => {
+    for (let j = 0; j < 10; j++) {
+      model.selectDigit('1');
+      model.selectOperation('*');
+      model.selectDigit(j.toString());
+      expect(model.expression).toBe('1*' + j.toString());
+      model = new CalculationModel();
+    }
+  })
+
+  it('should allow any digit after [ / ]', {}, () => {
+    for (let j = 0; j < 10; j++) {
+      model.selectDigit('1');
+      model.selectOperation('/');
+      model.selectDigit(j.toString());
+      expect(model.expression).toBe('1/' + j.toString());
+      model = new CalculationModel();
+    }
+  })
+
+  it('should allow any digit after [ . ]', {}, () => {
+    for (let j = 0; j < 10; j++) {
+      model.selectDigit('1');
+      model.selectDot();
+      model.selectDigit(j.toString());
+      expect(model.expression).toBe('1.' + j.toString());
+      model = new CalculationModel();
+    }
+  })
+
+  it('should allow any digit after [ ( ]', {}, () => {
+    for (let j = 0; j < 10; j++) {
+      model.selectLeftParen();
+      model.selectDigit(j.toString());
+      expect(model.expression).toBe('(' + j.toString());
+      model = new CalculationModel();
+    }
+  })
+
+  it('should not allow any digit after [ ) ]', {}, () => {
+    for (let j = 0; j < 10; j++) {
+      model.selectLeftParen();
+      model.selectDigit('1');
+      model.selectRightParen();
+      model.selectDigit(j.toString());
+      expect(model.expression).toBe('(1)');
+      model = new CalculationModel();
+    }
+  })
+})
+
+describe('Input [ + - * / ] in the middle of expression', {}, () => {
+  let model;
+  beforeEach(() => {
+    model = new CalculationModel();
+  })
+
+  it('should allow [ + ] after any digit', {}, () => {
+    for (let j = 0; j < 10; j++) {
+      model.selectDigit(j.toString());
+      model.selectOperation('+');
+      expect(model.expression).toBe(j.toString() + '+');
+      model = new CalculationModel();
+    }
+  })
+
+  it('should allow [ - ] after any digit', {}, () => {
+    for (let j = 0; j < 10; j++) {
+      model.selectDigit(j.toString());
+      model.selectOperation('-');
+      expect(model.expression).toBe(j.toString() + '-');
+      model = new CalculationModel();
+    }
+  })
+
+  it('should allow [ * ] after any digit', {}, () => {
+    for (let j = 0; j < 10; j++) {
+      model.selectDigit(j.toString());
+      model.selectOperation('*');
+      expect(model.expression).toBe(j.toString() + '*');
+      model = new CalculationModel();
+    }
+  })
+
+  it('should allow [ / ] after any digit', {}, () => {
+    for (let j = 0; j < 10; j++) {
+      model.selectDigit(j.toString());
+      model.selectOperation('/');
+      expect(model.expression).toBe(j.toString() + '/');
+      model = new CalculationModel();
+    }
+  })
+
+  it('should allow [ + ] after [ . ]', {}, () => {
+    model.selectDigit('1');
+    model.selectDot();
+    model.selectOperation('+');
+    expect(model.expression).toBe('1.+');
+  })
+
+  it('should allow [ - ] after [ . ]', {}, () => {
+    model.selectDigit('1');
+    model.selectDot();
+    model.selectOperation('-');
+    expect(model.expression).toBe('1.-');
+  })
+
+  it('should allow [ * ] after [ . ]', {}, () => {
+    model.selectDigit('1');
+    model.selectDot();
+    model.selectOperation('*');
+    expect(model.expression).toBe('1.*');
+  })
+
+  it('should allow [ / ] after [ . ]', {}, () => {
+    model.selectDigit('1');
+    model.selectDot();
+    model.selectOperation('/');
+    expect(model.expression).toBe('1./');
+  })
+
+  it('should not allow [ + ] after [ ( ]', {}, () => {
+    model.selectLeftParen();
+    model.selectOperation('+');
+    expect(model.expression).toBe('(');
+  })
+
+  it('should not allow [ * ] after [ ( ]', {}, () => {
+    model.selectLeftParen();
+    model.selectOperation('*');
+    expect(model.expression).toBe('(');
+  })
+
+  it('should not allow [ / ] after [ ( ]', {}, () => {
+    model.selectLeftParen();
+    model.selectOperation('/');
+    expect(model.expression).toBe('(');
+  })
+
+  it('should allow [ - ] after [ ( ]', {}, () => {
+    model.selectLeftParen();
+    model.selectOperation('-');
+    expect(model.expression).toBe('(~');
+  })
+
+  it('should allow [ + ] after [ ) ]', {}, () => {
+    model.selectLeftParen();
+    model.selectDigit('1');
+    model.selectRightParen();
+    model.selectOperation('+');
+    expect(model.expression).toBe('(1)+');
+  })
+
+  it('should allow [ - ] after [ ) ]', {}, () => {
+    model.selectLeftParen();
+    model.selectDigit('1');
+    model.selectRightParen();
+    model.selectOperation('-');
+    expect(model.expression).toBe('(1)-');
+  })
+
+  it('should allow [ * ] after [ ) ]', {}, () => {
+    model.selectLeftParen();
+    model.selectDigit('1');
+    model.selectRightParen();
+    model.selectOperation('*');
+    expect(model.expression).toBe('(1)*');
+  })
+
+  it('should allow [ / ] after [ ) ]', {}, () => {
+    model.selectLeftParen();
+    model.selectDigit('1');
+    model.selectRightParen();
+    model.selectOperation('/');
+    expect(model.expression).toBe('(1)/');
+  })
+})
+
+describe('Input [ ( ) ] in the middle of expression', {}, () => {
+  let model;
+  beforeEach(() => {
+    model = new CalculationModel();
+  })
+
+  it('should allow [ ( ] after [ + ]', {}, () => {
+    model.selectDigit('1');
+    model.selectOperation('+');
+    model.selectLeftParen();
+    expect(model.expression).toBe('1+(');
+  })
+
+  it('should allow [ ( ] after [ - ]', {}, () => {
+    model.selectDigit('1');
+    model.selectOperation('-');
+    model.selectLeftParen();
+    expect(model.expression).toBe('1-(');
+  })
+
+  it('should allow [ ( ] after [ - ] (negation)', {}, () => {
+    model.selectOperation('-');
+    model.selectLeftParen();
+    expect(model.expression).toBe('~(');
+  })
+
+  it('should allow [ ( ] after [ * ]', {}, () => {
+    model.selectDigit('1');
+    model.selectOperation('*');
+    model.selectLeftParen();
+    expect(model.expression).toBe('1*(');
+  })
+
+  it('should allow [ ( ] after [ / ]', {}, () => {
+    model.selectDigit('1');
+    model.selectOperation('/');
+    model.selectLeftParen();
+    expect(model.expression).toBe('1/(');
+  })
+
+  it('should allow [ ( ] after [ ( ]', {}, () => {
+    model.selectLeftParen();
+    model.selectLeftParen();
+    expect(model.expression).toBe('((');
+  })
+
+  it('should not allow [ ( ] after any digit', {}, () => {
+    for (let i = 0; i < 10; i++) {
+      model.selectDigit(i.toString());
+      model.selectLeftParen();
+      expect(model.expression).toBe(i.toString());
+      model = new CalculationModel();
+    }
+  })
+
+  it('should not allow [ ( ] after [ ) ]', {}, () => {
+    model.selectLeftParen();
+    model.selectRightParen();
+    expect(model.expression).toBe('(');
+  })
+
+  it('should not allow [ ( ] after [ . ]', {}, () => {
+    model.selectDigit('1');
+    model.selectDot();
+    model.selectRightParen();
+    expect(model.expression).toBe('1.');
+  })
+
+  it('should not allow [ ) ] after [ + ]', {}, () => {
+    model.selectDigit('1');
+    model.selectOperation('+');
+    model.selectRightParen();
+    expect(model.expression).toBe('1+');
+  })
+
+  it('should not allow [ ) ] after [ - ]', {}, () => {
+    model.selectDigit('1');
+    model.selectOperation('-');
+    model.selectRightParen();
+    expect(model.expression).toBe('1-');
+  })
+
+  it('should not allow [ ) ] after [ - ] (negation)', {}, () => {
+    model.selectOperation('-');
+    model.selectRightParen();
+    expect(model.expression).toBe('~');
+  })
+
+  it('should not allow [ ) ] after [ * ]', {}, () => {
+    model.selectDigit('1');
+    model.selectOperation('*');
+    model.selectRightParen();
+    expect(model.expression).toBe('1*');
+  })
+
+  it('should not allow [ ) ] after [ / ]', {}, () => {
+    model.selectDigit('1');
+    model.selectOperation('/');
+    model.selectRightParen();
+    expect(model.expression).toBe('1/');
+  })
+})
+
+describe('Delete last input', {}, () => {
+  let model;
+  beforeEach(() => {
+    model = new CalculationModel();
+  })
+
+  it('test (1)', {}, () => {
+    model.expression = '1+2';
+    model.delete();
+    expect(model.expression).toBe('1+');
+    expect(model.leftParenCount).toBe(0);
+    expect(model.result).toBe('');
+  })
+
+  it('test (2)', {}, () => {
+    model.expression = '1+(';
+    model.leftParenCount = 1;
+    model.delete();
+    expect(model.expression).toBe('1+');
+    expect(model.leftParenCount).toBe(0);
+    expect(model.result).toBe('');
+  })
+
+  it('test (3)', {}, () => {
+    model.expression = '1+(3-2)';
+    model.leftParenCount = 0;
+    model.delete();
+    expect(model.expression).toBe('1+(3-2');
+    expect(model.leftParenCount).toBe(1);
+    expect(model.result).toBe('');
+  })
+
+  it('test (4): should delete nothing if expression is empty', {}, () => {
+    model.expression = '';
+    model.leftParenCount = 0;
+    model.delete();
+    expect(model.expression).toBe('');
+    expect(model.leftParenCount).toBe(0);
+    expect(model.result).toBe('');
+  })
+})
+
+describe('Evaluate valid expression', {}, () => {
+  let model;
+  beforeEach(() => {
+    model = new CalculationModel();
+  })
+
+  it('Basic Operations with Parentheses', {}, () => {
+    model.expression = '(10+5)*2-3/4';
+    model.leftParenCount = 0;
+    model.selectEvaluate();
+    expect(model.result.toString()).toBe('29.25');
+  })
+
+  it('Mixed Operations with Decimals', {}, () => {
+    model.expression = '(~1.5+3.5)-2*(0.5*6)';
+    model.leftParenCount = 0;
+    model.selectEvaluate();
+    expect(model.result.toString()).toBe('-4');
+  })
+
+  it('Nested Parentheses', {}, () => {
+    model.expression = '(((2+3)*4.)-5)/6';
+    model.leftParenCount = 0;
+    model.selectEvaluate();
+    expect(model.result.toString()).toBe('2.5');
+  })
+
+  it('Multiplication and Division Before Addition and Subtraction', {}, () => {
+    model.expression = '15-12/(6/2)*4+3';
+    model.leftParenCount = 0;
+    model.selectEvaluate();
+    expect(model.result.toString()).toBe('2');
+  })
+
+  it('Combination of All Operations', {}, () => {
+    model.expression = '~(10+5)*2-3./4+(~1.5+3.5)-2*(0.5*6)';
+    model.leftParenCount = 0;
+    model.selectEvaluate();
+    expect(model.result.toString()).toBe('-34.75');
+  })
+
+  it('Complex Expression with Multiple Operations', {}, () => {
+    model.expression = '72+4*6/2-8-((10+5)*2-3/4)';
+    model.leftParenCount = 0;
+    model.selectEvaluate();
+    expect(model.result.toString()).toBe('46.75');
+  })
+
+  it('Expression with Negation', {}, () => {
+    model.expression = '~(~10+5)*2.-3/4+(~1.5+3.5)';
+    model.leftParenCount = 0;
+    model.selectEvaluate();
+    expect(model.result.toString()).toBe('11.25');
+  })
+
+  it('Subtraction with Parentheses', {}, () => {
+    model.expression = '10-(5+3)*2/4';
+    model.leftParenCount = 0;
+    model.selectEvaluate();
+    expect(model.result.toString()).toBe('6');
+  })
+
+  it('Division Inside Parentheses', {}, () => {
+    model.expression = '10/(5+3)*2-3';
+    model.leftParenCount = 0;
+    model.selectEvaluate();
+    expect(model.result.toString()).toBe('-0.5');
+  })
+
+  it('Expression with Mixed Types and Operations', {}, () => {
+    model.expression = '72+4*6/2-8-((10+5)*2-3/4)+(~1.5+3.5)';
+    model.leftParenCount = 0;
+    model.selectEvaluate();
+    expect(model.result.toString()).toBe('48.75');
+  })
+
+  it('Deeply Nested Expressions', {}, () => {
+    model.expression = '((((10+5)*2-3/4)+((~1.5+3.5)*6))-(0.5*(2+3)))*4';
+    model.leftParenCount = 0;
+    model.selectEvaluate();
+    expect(model.result.toString()).toBe('155');
+  })
+
+  it('Complex Nested Structure with Decimals', {}, () => {
+    model.expression = '((10+5)*2-3/4)+((~1.5+3.5)*6)-(0.5*(2+3))+7-2';
+    model.leftParenCount = 0;
+    model.selectEvaluate();
+    expect(model.result.toString()).toBe('43.75');
+  })
+})
+
+describe('Evaluate incomplete expression returns null', {}, () => {
+  let model;
+  beforeEach(() => {
+    model = new CalculationModel();
+  })
+
+  it('test (1)', {}, () => {
+    model.expression = '3*';
+    model.leftParenCount = 0;
+    model.selectEvaluate();
+    expect(model.result).toBe(null);
+  })
+
+  it('test (2)', {}, () => {
+    model.expression = '3-';
+    model.leftParenCount = 0;
+    model.selectEvaluate();
+    expect(model.result).toBe(null);
+  })
+  it('test (3)', {}, () => {
+    model.expression = '~';
+    model.leftParenCount = 0;
+    model.selectEvaluate();
+    expect(model.result).toBe(null);
+  })
+
+  it('test (4)', {}, () => {
+    model.expression = '3*(';
+    model.leftParenCount = 1;
+    model.selectEvaluate();
+    expect(model.result).toBe(null);
+  })
+
+  it('test (5)', {}, () => {
+    model.expression = '3*(~';
+    model.leftParenCount = 1;
+    model.selectEvaluate();
+    expect(model.result).toBe(null);
+  })
+
+  it('test (6)', {}, () => {
+    model.expression = '3*(~2';
+    model.leftParenCount = 1;
+    model.selectEvaluate();
+    expect(model.result).toBe(null);
+  })
+
+  it('test (7)', {}, () => {
+    model.expression = '(';
+    model.leftParenCount = 1;
+    model.selectEvaluate();
+    expect(model.result).toBe(null);
+  })
+
+  it('test (8)', {}, () => {
+    model.expression = '5+(3.5+(6-4.5*(8/2)-4)-2.5';
+    model.leftParenCount = 1;
+    model.selectEvaluate();
+    expect(model.result).toBe(null);
+  })
+
+  it('test (9)', {}, () => {
+    model.expression = '5+(0.';
+    model.leftParenCount = 1;
+    model.selectEvaluate();
+    expect(model.result).toBe(null);
+  })
+
+  it('test (10)', {}, () => {
+    model.expression = '3*(~2+(';
+    model.leftParenCount = 2;
+    model.selectEvaluate();
+    expect(model.result).toBe(null);
+  })
+
+  it('test (11)', {}, () => {
+    model.expression = '3*(~2+(~';
+    model.leftParenCount = 2;
+    model.selectEvaluate();
+    expect(model.result).toBe(null);
+  })
+
+  it('test (12)', {}, () => {
+    model.expression = '~3*(~2+(~';
+    model.leftParenCount = 2;
+    model.selectEvaluate();
+    expect(model.result).toBe(null);
+  })
+
+  it('test (13)', {}, () => {
+    model.expression = '((5.5';
+    model.leftParenCount = 2;
+    model.selectEvaluate();
+    expect(model.result).toBe(null);
+  })
+
+  it('test (14)', {}, () => {
+    model.expression = '((5+';
+    model.leftParenCount = 2;
+    model.selectEvaluate();
+    expect(model.result).toBe(null);
+  })
+
+  it('test (15)', {}, () => {
+    model.expression = '((5-(6)';
+    model.leftParenCount = 2;
+    model.selectEvaluate();
+    expect(model.result).toBe(null);
+  })
+
+  it('test (16)', {}, () => {
+    model.expression = '~((5+(6)';
+    model.leftParenCount = 2;
+    model.selectEvaluate();
+    expect(model.result).toBe(null);
+  })
+
+  it('test (17)', {}, () => {
+    model.expression = '(~(5';
+    model.leftParenCount = 2;
+    model.selectEvaluate();
+    expect(model.result).toBe(null);
+  })
+})
+
+describe('Identify previous number when delete last input is selected', {}, () => {
+  let model;
+  beforeEach(() => {
+    model = new CalculationModel();
+  })
+
+  it('test (1)', {}, () => {
+    model.selectDigit('2');
+    model.selectDigit('3');
+    model.selectOperation('+');
+    model.delete();
+    expect(model.prevNumber).toBe('23');
+  })
+
+  it('test (2)', {}, () => {
+    model.selectDigit('2');
+    model.selectDigit('3');
+    model.selectDot();
+    model.selectOperation('*');
+    model.delete();
+    expect(model.prevNumber).toBe('23.');
+  })
+
+  it('test (3)', {}, () => {
+    model.selectDigit('2');
+    model.selectDigit('3');
+    model.selectDot();
+    model.selectDigit('4');
+    model.selectDigit('5');
+    model.selectOperation('*');
+    model.delete();
+    expect(model.prevNumber).toBe('23.45');
+  })
+
+  it('test (4)', {}, () => {
+    model.selectDigit('2');
+    model.selectDigit('3');
+    model.selectOperation('+');
+    model.selectLeftParen();
+    model.delete();
+    expect(model.prevNumber).toBe('');
+  })
+  
+  it('test (5)', {}, () => {
+    model.selectDigit('2');
+    model.selectOperation('+');
+    model.selectLeftParen();
+    model.selectDigit('3');
+    model.delete();
+    expect(model.prevNumber).toBe('');
+  })
+
+  it('test (6)', {}, () => {
+    model.selectDigit('2');
+    model.selectOperation('+');
+    model.selectLeftParen();
+    model.selectLeftParen();
+    model.selectDigit('3');
+    model.selectRightParen();
+    model.selectRightParen();
+    model.delete();
+    expect(model.prevNumber).toBe('');
+  })
+
+  it('test (7)', {}, () => {
+    model.selectDigit('2');
+    model.selectOperation('+');
+    model.selectDigit('3');
+    model.selectDigit('5');
+    model.delete();
+    expect(model.prevNumber).toBe('3');
+  })
+
+  it('test (8)', {}, () => {
+    model.selectDigit('2');
+    model.selectOperation('+');
+    model.selectDigit('3');
+    model.selectDigit('5');
+    model.selectDot();
+    model.delete();
+    expect(model.prevNumber).toBe('35');
+  })
+
+  it('test (9)', {}, () => {
+    model.selectDigit('2');
+    model.selectOperation('+');
+    model.selectDigit('3');
+    model.selectDigit('5');
+    model.selectDot();
+    model.selectDigit('6');
+    model.delete();
+    expect(model.prevNumber).toBe('35.');
+  })
+
+  it('test (10)', {}, () => {
+    model.selectDigit('2');
+    model.selectOperation('+');
+    model.selectDigit('3');
+    model.selectDigit('5');
+    model.selectDot();
+    model.selectDigit('6');
+    model.selectDigit('8');
+    model.delete();
+    expect(model.prevNumber).toBe('35.6');
+  })
+})