Two players A and B play a series of games of badminton. The player, who wins 5 games first, wins the series. Assuming that no game ends in a draw, the number of ways, in which player A wins the series is .
Answer
126
Solution
If A wins the series and B wins k games (k=0, 1, 2, 3, 4), the series has 5+k games and A wins the last one.
The first 4+k games contain 4 wins of A and k wins of B: 4+kCk ways
Number of ways = k=044+kCk=4C0+5C1+6C2+7C3+8C4
=1+5+15+35+70=126
Answer: 126