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.