Answer:
25 feet
Explanation:
Use the pythagorean theorem, where the width and length of the pool are the legs of the triangle.
Solve for c, the hypotenuse/diagonal
a² + b² = c²
15² + 20² = c²
225 + 400 = c²
625 = c²
25 = c
So, the length of the diagonal is 25 feet