Because the fence's dimensions is 30 feet by 10 feet, you can assume that one side of the pool is 30 feet while the other, non-congruent side, is 10 feet. The formula for perimeter, which you are trying to find, is 2L+2W=P. In this problem, 30 will be your L and 10 will be your W. Therefore, 2(30) +2(10)=80.
