Shuffle the cards and spread them out, being sure to memorize the NINTH card from the bottom.
Turn the deck face down and your memorized card is now ninth from the top.
Ask the spectator to pick a number from 10-19.
Deal down the number of cards she says. If she says 15, deal 15 cards face down.
Have her add the 2 digits of her number. If she picked 15, 1+5=6. Have her deal--from the previously dealt pile of 15--6 cards.
From the new pile of 6, have her memorize the top card. If she knows how to do simple math, it will be the card you memorized at the start of the trick. The way it works is that, with any number from 10-19, when you add the 2 digits together and then subtract that from the original number, you ALWAYS get 9. For instance, 14...1+4=5...14-5=9. With 17...1+7=8...17-8=9.
After she memorizes that card, put the entire deck together, shuffle, and spread the cards out across the table. Put your finger over them and look at her reaction to the cards, as dramatics, and then "figure out" what card she memorized.