If you love math or simply want to look cognitively enhanced! 

While you're about it think on this: 3 men have lunch for $30. Each pays $10. The manager realizes the bill should only be $25, so asks the waiter to return $5. Each man takes $1 and the waiter keeps $2. Each man has now paid $9. That comes to a total of $27. The waiter has $2. That's $29. Where's the missing $1?