Sally has christmas lights the roof is 16 feet high the ladder is 20 ft long how far away is letter from wall

It should be 12 ft away from the wall. Not 4 ft. You do not just subtract the difference because as you pull it away from the wall, the ladder is mostly moving horizontally with only a small vertical change.

You can use Pythagorean Theorem to find the answer. Think of the ladder as the hypotenuse of a right triangle with the wall and the ground as the legs.

a^2 + b^2 = c^2

16^2 + b^2 = 20^2

b^2 = 20^2 - 16^2

b^2 = 144

b = 12 ft