Explanation:
A.
15/5 = 3, 21/7 = 3 -- scale factor is 1/3 since he must divide by 3 to get the side lengths for the photo card
B.
Area = width*length, and scale factor is 5. Therefore, the new width is (15*5) and the new length is (21*5), so the new area is 15*5*21*5 = 15*21*5² = 15*21*25 = 315*25 = 7875 in²