Ramon wants to cut a rectangular board into identical square pieces. If the board is 18 inches by 30 inches, what is the least number of square pieces he can cut without wasting any of the board?

GRE Quant Problem Solving
1

Ramon wants to cut a rectangular board into identical square pieces. If the board is 18 inches by 30 inches, what is
the least number of square pieces he can cut without wasting any of the board?
(A) 4
(B) 6
(C) 9
(D) 12
(E) 15

I got it right till GCF then I didn’t understand how the answer became 15?

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3 posts were merged into an existing topic: How to solve this