Measurement Relationships: The Spider and the Fly Problem

The Spider and the Fly Problem is a classic puzzle that originally appeared in an English newspaper in 1903. It was posed by H.E. Dudeney. In a rectangular room with dimensions 30 ft by 12 ft by 12 ft, a spider is located in the middle of one 12 ft by 12 ft wall, 1 ft away from the ceiling. A fly is in the middle of the *opposite* wall 1 ft away from the floor. If the fly does not move, what is the shortest distance that the spider can crawl along the walls, ceiling and floor to capture the fly?

Hint: Using a net of the room will help you get the answer which is less than 42 ft!

I can't really get the right answer to this question it is supposed to be 40, any help would be appreciated!

Thanks!!