Create a new deck of the exactly same number of cards as are face up cards in the original deck.Take 15 number of cards in a new deck and change their face direction. For example- You create a new deck of 15 cards and out of 15, 5 faces up in a new deck. So remaining 10 faces up are in the old deck. But hey! while creating the new deck you reversed the face direction of new cards. So actually the 5 cards which were facing up are actually face down in the new deck while 10 faces up.