What is the gcf of 18 24 36?

Find the factors of all three numbers and then find the largest number that is in each list:

i.e. the factors of 18 are 1, 2, 3, 6, 9, 18.

So for example, to find the greatest common factor of 21, 28 and 42, find the factors of each:
Factors of 21 are 1,3,7,21
Factors of 28 are 1,2,4,7,14,28
Factors of 42 are 1,2,3,6,7,14,21,42

The highest number that is in all three lists is 7. Therefore, 7 is the greatest common factor.

Note: A factor is a number that divides evenly into the number. So, 2 divides evenly into 8 so 2 is a factor of 8. 3 divides evenly into 9 so 3 is a factor of 9, etc.