50 ounces of 15% saline solution contains 0.15*50 = 7.5 ounces of salt.
Let x be the amount of pure water you add to this solution. Then you end up with (x + 50) ounces of this new solution. The amount of salt stays the same.
You want the new solution to have a concentration of 10%. This means you need
First compute the perimeter of the pool: (21+33)*2=108 foot. Since the material cover 232 ft^2, then it is sufficient to divide the above number over the length of the border like this: 232/108=2.1
