Before the performance: remember what the ninth card of a card is, turn this card into the head of another deck (everything else is reversed) and put it in the box.

Performance: Take out the card (the first card) that you remember, fake-wash (or not), let the audience randomly choose a number from 10 to 20, then you count so many cards (one at a time), and then let the audience add up the digits of the number he chose, telling you that you can take the first card number and count so many cards (. Mysteriously said, "I knew you would choose this card." ...................................................................................................................................................................

Mathematical principle: Let the cards chosen by the audience be 10+y, 10+y-( 1+y)=9 (so it must be the ninth card).