A box has b black balls and r red balls. Balls are drawn from the box one at a time without replacement. Find the probability that the first black ball selected is drawn at the nth trial. The following problem pertains to poker hands. A deck has 52 cards. These cards consist of 4 suits called clubs, diamonds, hearts, and spades. Each suit has 13 cards labeled 2, 3, … , 10, J, Q, K, A. A poker hand consists of 5 cards drawn without replacement and without regard to order from the deck. Poker hands of the following types are considered to be in sequence: A, 2, 3, 4, 5; 2, 3, 4, 5, 6; . … ; 10, J, Q, K, A.