Here we have a rectangular room, measuring 30 feet by 12 feet, and 12 feet high.

There is a spider in the middle of one of the end walls, 1 foot from the ceiling (A).

There is a fly in the middle of the opposite wall, 1 foot from the floor (B).

What is the shortest distance that the spider must crawl in order to reach the fly?

The Spider and the Fly – The Canterbury Puzzles, Henry Ernest Dudeney.