The answer is 19.4
Most of the time when you try to find a missing length of a right triangle you use Pythagorean theorem. Pythagorean theorem is A squared plus B squared equals C squared. C squared is always going to be the longest length/diagonal line otherwise known as your hypotenuse. In this case the ladder is the hypotenuse. So instead of adding five squared +20 squared you’re going to square 20 and then subtract five squared to get the missing length which would be B squared and then you have to find the square root of B squared. So 20 squared is 4000. 5squared is 25 400-25 = 375 square root of 375 is about 19.4.