This creates a right triangle. One leg of the triangle is 25 feet and the other is 10 feet. To find the length of the ladder, we must use Pythagorean theorem.
a = 25
b = 10
c = ?
(25)^2 + (10)^2 = c^2. Square all the terms to the left of the equation and add them together.
725 = c^2. Take the square root of each side.
c = 26.93 feet.
The ladder needs to be 26.93 feet tall.