**Definition of Highest Common Factor (HCF)**

The Highest Common Factor (HCF) of two or more numbers is the highest number that divides the numbers exactly.

**More about Highest Common Factor (HCF)**

If the highest common factor (HCF) of two numbers is equal to 1, then they are called co-prime or relatively prime.

The product of the highest common factor (HCF) and the lowest common multiple (LCM) of two numbers is equal to the product of the numbers.

**Example of Highest Common Factor (HCF)**

To find the HCF of 12, 24, and 36, first we list out the factors of the three numbers.

12 - 1, 2, 3, 4, 6, and **12**

24 - 1, 2, 3, 4, 6, 8, **12**, and 24

36 - 1, 2, 3, 4, 6, 8, **12**, 18, and 36

So, the HCF of the numbers 12, 24, and 36 is 12.

In another method we need to write all the prime factors of the three numbers 12, 24, and 36.

12 - 2 × 2 × 3

24 - 2 × 2 × 2 × 3

36 - 2 × 2 × 3 × 3

Then we list out all the common prime factors. The common prime factors of the three numbers are 2 × 2 × 3.

Then we have to multiply the common prime factors.

2 × 2 × 3 = 12

So, the HCF of the three numbers 12, 24, and 36 is 12.

**Solved Example on Highest Common Factor (HCF)**

Find the HCF of 18 and 35.

**Choices:**

A. 1

B. 5

C. 7

D. 9

**Correct Answer: A**

**Solution:**

**Step 1:** 18 = 1, 2, 3, 6, 9, and 18 [List the factors of 18.]

**Step 2:** 35 = 1, 5, 7, and 35 [List the factors of 35.]

**Step 3:** The common factor of 18 and 35 is 1.

**Step 4: **So, the HCF of 18 and 35 is 1.

**Related Terms for Highest Common Factor (HCF)**

Common Factor

Factor

Number

Prime Factorization