To apply the Pythagorean Theorem, we can use the right triangle formed by the slant height, the height, and half of the base. Let's call the length of half the base b.
Using the Pythagorean Theorem, we have:
b^2 + h^2 = slant height^2
Since half the base is 40/2 = 20 feet, we have:
20^2 + h^2 = 25^2
400 + h^2 = 625
h^2 = 225
Taking the square root of both sides, we have:
h = 15 feet
So, the height of the square pyramid is 15 feet.
The correct answer is: 15.0 ft.