Question

Which expression represents the greatest common factor (GCF) of 30 and 80? A. 2 X 5 B. 2 X 7 C. 7 X 5 D. 2 X 2 X 5

Answer 1

30 = 5*6  = 5 * 3*2

80 = 8* 10 = 2*2*2 * 2 *5

The CGF is the numbers that appear in both  

2 appears 1 time in both and 5 appears 1 time in both

GCF 2*5

Choice A

Answer 2

So, you first need to find the GCF of 30 and 80.

80 - 1,2,4,5,8,10,16,20,40,80

30 - 1,2,3,5,6,10,15,30

Find the highest number that can go with both of these numbers.

The highest factor that can multiply to both of these numbers is 10.

Now, find the answer that has the right multiples to 10.

10 - 1,2,5,10.

The only answer with the right multiples matching up with the factors is         A. 2 × 5

So, your answer is A. 2 × 5