Brainteaser Day

Here’s a second chance to win a copy of one of the Professor Sophie Knowles mysteries. You may remember that Sophie, who teaches math at a small New England college, loves to solve puzzles and to create them.

Try this one:

A woman is stuck on an island. The island is surrounded by man-eating sharks and a single bridge is the only option to return to the mainland. Halfway across the bridge there’s a guard. The guard won’t let anyone from the mainland to the island, or anyone from the island to the mainland. If the guard catches someone, he sends him or her back. All day and night the guard sleeps for 30 seconds and then is awake for 5 minutes. It takes 1 minute to cross the bridge.

The woman thinks of a way to get across. How does she cross the bridge without getting caught?

The first three to email me with the/a correct answer will get a copy of a Sophie book.


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4 Responses to “Brainteaser Day”


    …Orrrr, she wears a reversible top and a wig/hat and starts crossing the bridge 5 seconds before nap time. Whether the guard sees her or not, as soon as he nods off she proceeds across while reversing her top and removing the wig/hat. After 33 seconds (28 nap seconds) she is 3 seconds onto the other side. So she turns around and moves toward the guard, who wakes up 2 seconds later and sees her coming from the mainland side! He doesn’t recognize her as the island escapee, so he chases her “back” onto the mainland!

  2. Ellen Smart says:

    While the troll is asleep the woman runs halfway across the bridge then faces backwards. She appears like she is running to the island and is sent to the mainland — troll guards the bridge, doesn’t say anything about purpose of the guarding, to keep in or out.


    In the dark she sneaks up, timing it so the guard falls asleep just before they meet (doesn’t matter whether the guard waits for her or moves toward her). Then with her 30 second head start she makes it across before he can catch her.
    (Alternate solution: She swims, since the sharks only eat men.)

  4. Paul tucker says:

    Actually I just tenet bet I’ve seen this before.
    She runs halfway and turns around so the guard thinks she is coming from the mainland and sends her back