This was an SAT question of the day, but I can't figure out how to come up with the right answer, I think you use Pythagoras Theorum but I'm not sure. Can anyone help with the process here?

Question

A 25-foot ladder is placed against a vertical wall of a building, with the bottom of the ladder standing on concrete 7 feet from the base of the building. If the top of the ladder slips down 4 feet, then the bottom of the ladder will slide out ___ ft.

Answer 1

Nevermind I figured it out!

Answer 2

Before the slipping occurs, the top of the ladder's distance above the floor is
sqrt(25^2 - 7^2) = 24 feet
If the top slips 4 feet, it then is 20 feet from the floor.
Now use the Pythagorean theorem again to get the distance of the base from the vertical wall.
sqrt (25^2 - 20^2) = __?__

Answer 3

15