What is the greatest common factor of 30 and 72

30  =  2 x 3 x 5

72  =  2 x 2 x 2 x 3 x 3

The greatest common factor is the largest number that can divide into both of these numbers. Since there is a 2 and a 3 in both numbers, the greatest common factor is 2 x 3 = 6.