4. (2 points) You and I play a coin-tossing game: if the coin falls heads I score one, if tails you score one. In the beginning, the score is zero. Assume the coin is fair. (a) (1 point) What is the probability that after 2n throws our scores are equal? (b) (1 point) What is the probability that after 2n + 1 throws my score is three more than yours?