Now, I hope you have drawn a diagram, let y = 0 be the bottom of the pool, so that y = 5 is at the top. We will compute the work needed to move an infinitesimal slice of water, of thickness , at a level above the bottom of the pool, and then integrate to find the total work.
Now, , where is force and is distance.
Note that the distance we want to move this slice will be , it should be easy to see this if you drew your diagram correctly.
Now, the force is
So that we have
now, the total work is the sum of the works needed to move all the infinitesimal slices, ranging from a level y = 0 to y = 4 of the pool, hence,
And note that the answer will be in ft-lbs