A 12-ft-by-15-ft rectangular swimming pool has a 3-ft-wide-no-slip surface around it. What is the outer perimeter of the no-slip surface?

Question

78 ft
78 ft^2
198 ft
198 ft^2

I've already completed it, and know the answer is 78 ft, but I don't quite understand it. I would appreciate if someone could explain the problem. I thought it would have been +3 to each side, so 12 ft becomes 15 ft, and the other side, which is 15 ft, will be 18 ft. This is incorrect though, so I'm confused.

Thank you!

Answer 1

it's 3 ft on each side (left&right, top&bottom), so 12 becomes 18, 15 becomes 21.

2(18+21) = 78

Answer 2

Thank you for the explanation.