Ask your dad for $50 and you asked your mom for $50. they both gave it, so now you have $100. you buy a coat for $97. so you have $3 left. you give one dollar to your mom, one dollar to your dad and you keep the other dollar. now you still have to pay your mom $49 and your dad $49 but... 49+49=98. plus the one dollar you kept is $99. what happened to the last dollar