Solve Math Puzzle

A fellow’s wife was very worried about her husband’s heavy drinking and one night she decided to give him a fright. She draped herself in a white sheet and went down to the local cemetery, knowing that her husband was in the habit of taking a shortcut through it on his way home from the pub. It was not long before he came staggering along, and out she jumped from behind a headstone.

“Ooooooo!” she wailed, “I am the devil!”

Her husband sticks out his hand. “Put it there, pal,” he says, “I’m married to your sister.”