Same Number

Take out 30 cards and deal them into 2 deck. Each deck contains 15 cards.
Then, put 2 deck of cards on top of each other and tell your specator to shuffle it carefully and don't drop the cards. If the cards dropped, do it all again. Now, you deal the cards into 2 deck once more, 15 cards in each deck. You can tell the spectator to do this part. Then, take both of the deck and put it behind your back. The 1st deck of cards, you just hold on your right hand. While the other hand holds the 2nd deck of cards. But the 2nd deck of cards must be turn over whole without spectator knowing it.

Take out both deck of cards and tell your spectator that both deck of cards contains the same number of backward-facing cards. If the 1st deck has 13 cards facing backwards, then the 2nd deck must contain the same number of cards too!

ENJOY THE TRICK!!

I didn't really get it...sorry