Three closed boxes have either white marbles, black marbles or both, and they are labeled white, black and both. However, you’re told that each of the labels are wrong. You may reach into one of the boxes and pull out only one marble. Which box should you remove a marble from to determine the contents of all three boxes?
The one labeled both. Since you know it’s labeled incorrectly, it must have all black marbles or all white marbles. After you determine what it contains, you can identify the other two boxes by the process of elimination.
As a result of temporary magical powers, you have made it to the Wimbledon finals and are playing Roger Federer for all the marbles. However, your powers cannot last the whole match. What score do you want it to be when they disappear, to maximize your chances of hanging on for a win?
It sounds obvious that you should ask to be ahead two sets to love (it takes 3 out of 5 sets to win
the men’s), and in the third set, ahead 5-0 in games and 40-love in the sixth game. (Probably you
want to be serving, but if your serve is like mine, you might prefer Roger to be serving the sixth
game down 0-40 so that you can pray for a double fault.)
Not so fast! These solutions give you essentially 3 chances to get lucky and win, but you can
get six chances—with three services by you and three by Roger. You still want to be up two sets
to none, but let the game score be 6-6 in the third set and 6-0—in your favor, of course—in the
tiebreaker.