diff --git a/08_calculator/calculator.js b/08_calculator/calculator.js
index c22e8d2..4712322 100644
--- a/08_calculator/calculator.js
+++ b/08_calculator/calculator.js
@@ -1,26 +1,32 @@
-const add = function() {
-	
-};
+const add = function (...numbers) {
+  return numbers.reduce((res, cur) => res + cur, 0)
+}
 
-const subtract = function() {
-	
-};
+const subtract = function (...numbers) {
+  return numbers.reduce((res, cur) => res - cur)
+}
 
-const sum = function() {
-	
-};
+const sum = function (numbers) {
+  return add(...numbers)
+}
 
-const multiply = function() {
+const multiply = function (...numbers) {
+  return numbers.reduce((res, cur) => res * cur)
+}
 
-};
+const power = function (...numbers) {
+  return numbers.reduce((res, cur) => res ** cur)
+}
 
-const power = function() {
-	
-};
-
-const factorial = function() {
-	
-};
+const factorial = function (number) {
+  if (number < 0) {
+    return -1
+  } else if (number === 0) {
+    return 1
+  } else {
+    return number * factorial(number - 1)
+  }
+}
 
 // Do not edit below this line
 module.exports = {
@@ -29,5 +35,5 @@ module.exports = {
   sum,
   multiply,
   power,
-  factorial
-};
+  factorial,
+}
diff --git a/08_calculator/calculator.spec.js b/08_calculator/calculator.spec.js
index 3507452..175c681 100644
--- a/08_calculator/calculator.spec.js
+++ b/08_calculator/calculator.spec.js
@@ -1,77 +1,77 @@
-const calculator = require('./calculator');
+const calculator = require('./calculator')
 
 describe('add', () => {
-	test('adds 0 and 0', () => {
-		expect(calculator.add(0,0)).toBe(0);
-	});
+  test('adds 0 and 0', () => {
+    expect(calculator.add(0, 0)).toBe(0)
+  })
 
-	test.skip('adds 2 and 2', () => {
-		expect(calculator.add(2,2)).toBe(4);
-	});
+  test('adds 2 and 2', () => {
+    expect(calculator.add(2, 2)).toBe(4)
+  })
 
-	test.skip('adds positive numbers', () => {
-		expect(calculator.add(2,6)).toBe(8);
-	});
-});
+  test('adds positive numbers', () => {
+    expect(calculator.add(2, 6)).toBe(8)
+  })
+})
 
 describe('subtract', () => {
-	test.skip('subtracts numbers', () => {
-		expect(calculator.subtract(10,4)).toBe(6);
-	});
-});
+  test('subtracts numbers', () => {
+    expect(calculator.subtract(10, 4)).toBe(6)
+  })
+})
 
 describe('sum', () => {
-	test.skip('computes the sum of an empty array', () => {
-		expect(calculator.sum([])).toBe(0);
-	});
+  test('computes the sum of an empty array', () => {
+    expect(calculator.sum([])).toBe(0)
+  })
 
-	test.skip('computes the sum of an array of one number', () => {
-		expect(calculator.sum([7])).toBe(7);
-	});
+  test('computes the sum of an array of one number', () => {
+    expect(calculator.sum([7])).toBe(7)
+  })
 
-	test.skip('computes the sum of an array of two numbers', () => {
-		expect(calculator.sum([7,11])).toBe(18);
-	});
+  test('computes the sum of an array of two numbers', () => {
+    expect(calculator.sum([7, 11])).toBe(18)
+  })
 
-	test.skip('computes the sum of an array of many numbers', () => {
-		expect(calculator.sum([1,3,5,7,9])).toBe(25);
-	});
-});
+  test('computes the sum of an array of many numbers', () => {
+    expect(calculator.sum([1, 3, 5, 7, 9])).toBe(25)
+  })
+})
 
 describe('multiply', () => {
-	test.skip('multiplies two numbers', () => {
-		expect(calculator.multiply(2,4)).toBe(8);
-	});
+  test('multiplies two numbers', () => {
+    expect(calculator.multiply(2, 4)).toBe(8)
+  })
 
-	test.skip('multiplies several numbers', () => {
-		expect(calculator.multiply(2,4,6,8,10,12,14)).toBe(645120);
-	});
-});
+  test('multiplies several numbers', () => {
+    expect(calculator.multiply(2, 4, 6, 8, 10, 12, 14)).toBe(645120)
+  })
+})
 
 describe('power', () => {
-	test.skip('raises one number to the power of another number', () => {
-		expect(calculator.power(4,3)).toBe(64); // 4 to third power is 64
-	});
-});
+  test('raises one number to the power of another number', () => {
+    expect(calculator.power(4, 3)).toBe(64) // 4 to third power is 64
+  })
+})
 
 describe('factorial', () => {
-	test.skip('computes the factorial of 0', () => {
-		expect(calculator.factorial(0)).toBe(1); // 0! = 1
-	});
+  test('computes the factorial of 0', () => {
+    expect(calculator.factorial(0)).toBe(1) // 0! = 1
+  })
 
-	test.skip('computes the factorial of 1', () => {
-		expect(calculator.factorial(1)).toBe(1);
-	});
+  test('computes the factorial of 1', () => {
+    expect(calculator.factorial(1)).toBe(1)
+  })
 
-	test.skip('computes the factorial of 2', () => {
-		expect(calculator.factorial(2)).toBe(2);
-	});
+  test('computes the factorial of 2', () => {
+    expect(calculator.factorial(2)).toBe(2)
+  })
 
-	test.skip('computes the factorial of 5', () => {
-		expect(calculator.factorial(5)).toBe(120);
-	});
+  test('computes the factorial of 5', () => {
+    expect(calculator.factorial(5)).toBe(120)
+  })
 
-	test.skip('computes the factorial of 10', () => {
-		expect(calculator.factorial(10)).toBe(3628800);
-	});
-});
+  test('computes the factorial of 10', () => {
+    expect(calculator.factorial(10)).toBe(3628800)
+  })
+})