secret Wrong prediction

the megician fans out 15 cards from the top of deck and asks you to select any number of them from any location you are asked to keep them he then places 20 cards in a line on the table in front of you.
You are asked to count how many cards you have in your hand, the megician the proceds to count the same number along the line and gives you the card he stops on. You are asked to remember this card and all the cards are put back into a single stock.
The megiciao then tells your if he cannot guess your card that he will buy you a drink. The megician tells you your card but it's wrong,has a second attempt and gets it wrong once again a third and final attempt also falls! The megician open his wallet and there is no money in it, just your selected card!
There are several things to prepare for this trick the first is the selected card let's say the 5 of heart, it must be the 21st card in the deck from the top an extras of hearts must be in your wallet. When you count the 20 cards onto the table lay them down right to left and when you count back the volunteers card ammount count it from the left this will ensure that your volunter will always select the 5 of hearts trust me!