My "A Little Mystery" prompted a number of emails explaining how arithmetic only appeared to be broken. One person hinted that I might not understand the minor mystery. Most explanations were clear and concise. My story is an old math word problem dressed up a bit giving the salesmen an excuse for renting a motel.
Several people pointed out the misdirection. The arithmetic is correct as it stands but it has no reason to sum to 30. Try running the story with Billy not returning the $5. 30+5 vs the story's 27+2.