Take out two coins from your pocket, one for one yuan and one for fifty cents. Give the coin to a friend and ask her to put it in her right hand and left hand behind her. Then let her multiply the number of coins in her left hand by 55 and the number of coins in her right hand by 66. You can confirm the coin in her girlfriend's hand by looking at her expression.

Tip: It takes a long time to do the operation on a dime, so it's easy to distinguish it.