Highest Common Factor
The Highest Common Factor (HCF) of two or more given numbers is the highest (or greatest) of their common factors. 
It is also known as Greatest Common Divisor (GCD). 
The HCF of 20, 28 and 36 can also be found by prime factorisation of these numbers as follows:

The common factor of 20, 28 and 36 is 2(occuring twice). Thus, HCF of 20, 28 and 36 is 2 × 2 = 4.