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.
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.