13a. A box contains four red balls and two white balls. Darren and Marty play a game by each taking it 15N.1 in turn to take a ball from the box, without replacement. The first player to take a white ball is the winner. Darren plays first, find the probability that he wins. 13b. The game is now changed so that the ball chosen is replaced after each turn. Darren still plays first. Show that the probability of Darren winning has not changed.