Man 2: two minutes to cross

Man 3: five minutes to cross

Man 4: 10 minutes to cross

The bridge will be blown up in 17 minutes. How can they all cross in time?

(Note: The bridge is too long for the flashlight to be thrown across it. The flashlight cannot shine a long distance. No one can be carried. There are no tricks.)

THE ANSWERS

1. The person with the gold puts the pot into his box. He locks it, but keeps his key. He sends it over with the boatman. The other one then puts his lock on the box and keeps his key. Then he sends it back to the first person. Then the first person removes his lock and sends it back to his friend. The second person unlocks the box, opens it and revels in his newfound wealth.

2. Do this one by factors. The first gem has only been touched once, because one only has itself as a factor. The second one has been touched twice, because two has two factors. In fact, all of the numbers have an even number of factors, except for the perfect squares. So only the gems in the positions of perfect squares will kill you.

3. The circle is the only shape that can't be tilted or rotated to fit into itself. Since we don't want our manholes to accidentally fall through the holes made for them, the round shape works best.

4. 7.5 degrees. When the minute hand reaches 3:00, the hour hand has already moved on toward 4:00. Since 15 minutes is a quarter of an hour, it has moved a quarter of the way from 3:00 to 4:00, which is a twelfth of 360 degrees.

5. Pick from the box labeled "mixed." The label is wrong, so the color of the ball is the color of all the balls in the box. The box with the label of the other color ("red" or "blue") must contain the mixed balls, since otherwise its label would be correct, and the remaining box must contain the balls of the other color.

6. There are 1,000 small cubes in the larger one. The part of the cube that touches no surface is 8 by 8 by 8, so it contains 512 unit cubes. Therefore 1000 - 512 = 488 unit cubes touch at least one surface.

7. Ask either of the two which road the other person would tell you to take to get to Harvard, and then take the opposite road. The liar will always point to the road to Yale, either because he is a liar or because he would lie about what the truth-teller would say. Since no matter who you ask, the liar has a part in the answer, you will always be told the road to Yale.

8. Fill the three-cup flask with water. Dump it into the seven-cup flask. Then refill the three-cup flask and dump it into the seven-cup flask. There now is one cup of water in the three-cup flask. Next, dump the water out of the seven-cup flask and fill it with water. Pour it into the three-cup flask up to the rim. There now is five cups of water left in the seven-cup flask.

9. First, the first man and the second man walk across, using up two minutes. Then the second man walks back with the flashlight, using up another two minutes. Then the third man and the fourth man walk across, using ten minutes. The first man walks back using one minute. The first man and the second man walk across using up two minutes. 2 + 2 + 10 + 1 + 2 = 17 minutes.