HCF /GCF : Highest common factor or greatest common factor : The highest number which is factor of all given numbers is known to be HCF of given numbers.

HCF of 15 & 20

Factors of 15 = 3 x 5

Factors of 20 = 2 x 2 x 5. Means we can see only number 5 is common between factors of both the numbers so 5 is HCF of 15 and 20.

Facts :

1. HCF of two consecutive numbers will always be 1.

2. HCF of two prime number will always be 1.

3. For two numbers : HCF X LCM = Product of the numbers.