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.