16. A deck of 52 playing cards is shuffled and the cards are then turned face up one at a time. Let equal 1 if the ith card turned over is an ace, and let it be 0 otherwise,I mage. Also, let N denote the number of cards that need be turned over until all four aces appear. That is, the final ace appears on the Nth card to be turned over. Is the equation valid? If not, why is Wald's equation not applicable?