Suppose that two evenly matched teams (say team A and team B), each with probability .5 of winning any given game, make it to the baseball World Series. The series ends as soon as one of the teams has won four games. Thus, it can end as early as the 4 th game (a "sweep") or as late as the 7 th game, with one team winning its fourth game after having lost three of the ?rst six games. Calculate the probability that the series ends in 4 games, 5 games, 6 games, or 7 games.