Answer: 18 feet
Reason: he swam 9 feet going down the pool, but he didn’t just stay under there, he swam back up to the surface.
Thus we can 9 (feet for swimming to the bottom) plus 9 ( feet for swimming to the top).
In mathematical term we can say this in two ways.
9 + 9 = 18
Or
9 x 2 = 18
