The magician shows the audience two decks of playing cards and asks the audience to mix them together and take out a stack of cards at will (but the number of cards should not be less than 10). Count the cards with your back to the magician, but don't tell the magician. Then add the ten digits and the single digits of the number of cards to get a sum number. Don't tell the magician this number, but the audience needs to take out the number of cards with this sum number from a stack of cards (if the original card is folded into 35 cards and the sum number is 8, take 8 cards), and then give the remaining cards to the magician. When the magician takes a look at poker, he can immediately tell how many cards are in the stack.

A principle in applied mathematics, that is, any two-digit minus the sum of its 10 digit and a digit, their difference must be a multiple of 9. For example:

15-( 1+5)=9= 1×9

36-(8+6)=27=8×9

57-(5+7)=45=5×9

74-(7+4)=63=7×9 In short, the cards left by the above method must be 9, 18, 27, 36, 45, 63, 72, 8 1. The magician looked at the cards in his hand and found that they matched the numbers on them.

But the magician must always look at the playing cards and tell them whether there are nine more cards or nine less cards. If you think 45 is 36 or 64, you are joking. In addition, if you only use a deck of poker, the numbers will be more accurate and easier to execute.