The magician took out a deck of playing cards and confidently said to the audience, "all you have to do is draw a stack from this deck of playing cards." I just need to weigh it with my hand and know how many cards there are. " "

The audience representatives randomly drew a stack of playing cards, and everyone quietly counted 43 cards.

The magician said, "Please add up the ten digits and the single digits of this stack of playing cards, and then take out the number of playing cards from this stack. I'll weigh it again."

The audience calculated according to the magician's requirements: 43 playing cards, the sum of ten digits and single digits is 4+3=7, 43-7=36. So the audience representative drew seven cards and gave the rest to the magician.

The magician put the remaining playing cards in his hand and weighed them gently. He immediately said, "This stack of cards is ***36."

Principle:

Suppose ab cards are taken from a pile of playing cards (A is ten digits and B is single digits), which can be listed as an equation according to the regulations:

10×a+b-(a+b)=9×a

(Number of sheets taken out-ten digits and single digits and number of sheets taken out)

The final result is that the number is 9×a, which means that the sum of the number on the natural number minus its digits must be a multiple of 9.