Greatest common factor or 60 and 20

To find the greatest common factors of these 2 numbers, first write them as product of prime numbers.

(a prime number is a number whose only factors are 1 and themselves, for example: 2, 3, 11 ...etc)

To write the numbers as product of prime factors, just divide them continuously until all factors are primes:



The greatest common factor, must contain the largest number of common factors found in both numbers which are 2- 2's and one 5

So the greatest common factor is 

20

It would be 20, because the actual number of one of them is divisible by both.

If you want a list of all the factors:

60: 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, 60

20: 1, 2, 4, 5, 10, 20

And the greatest one they share is 20.