I'm not sure if this is right, but the system works. Their original bill is $30 which split 3 ways is $10, but their later bill of $25 means that each owes $8.33, meaning they've each overpaid by about $1.67. Each receive $1 back which means they've overpaid by $0.67. $0.67x3 = $2 which is what the waiter took. That's as much as I can figure out. -____- I give up.