Let's say we're at the store and you want to buy 10 boxes of cookies for a party. Each box of cookies cost 4.89(counting tax), you don't know if you have enough money to buy the cookies with the $65.00 you have with you. You could round the price up to $5.00 and do more simple multiplication. Your total would estimate about $50.00. Now you know you have more than enough money to buy cookies. I hope that makes sense. :)