Seems like the puzzle collections from the Martin Gardner books is unlimited, and also of outstanding quality. While flipping through one of the books (Mathematical Puzzles and Diversions), I found another gem in Chapter 12. Some people have told me that they get scared by the complexity of these puzzles, and hence an upfront clarification, that the puzzle below does not require any algebra, just simple (and yet tricky) logic. Please try it, and make your children try it.
Two ferryboats start at the same instant from opposite sides of the river travelling across the water en route at right angles to the shores. Each travels at a constant speed but one is faster than the other. They pass at a point 720 meters from the nearest shore. Both boats remain at their slips for ten minutes before starting back. On the return trip, they pass 400 meters from the other shore. How wide is the river?
Al always, please send your answers directly to me at firstname.lastname@example.org. If you like the puzzle, please share it with others. If you have interesting puzzles to share, please send them to me at my e-mail given above.