You will see that the ladder, tree and ground form a right-angled triangle and so we can use Pythagoras' Theorem: c^2 = a^2 + b^2, where c is the hypotenuse (in this case the length of the ladder) and a and b are side lengths (in this case the distance between the tree and the ladder and the height that the ladder goes up the tree)
c^2 = a^2 + b^2
20^2 = 12^2 + b^2
400 = 144 + b^2
b^2 = 256
b = 16 feet
