An old chestnut
goes like this:
In an alley, with a horizontal ground and vertical walls on each side, there are two ladders, each with one end wedged in a corner and the end leaning against a wall. The ladders are of lengths A and B and they cross at distance h above the ground. How wide is the alley?
(This problem appears in many variations with different distances. The classic version uses ladders of lengths 20 and 30 which cross at a distance 10 above the ground, but many other sets of numbers have been used.)