The Underpass

A large truck had become wedged under an underpass. The driver couldn’t move it backward or forward, and traffic was beginning to back up behind it.

He was on the point of desperation when a little boy approached him and offered a suggestion. A few minutes later the truck was on its way. What did the boy tell the driver?

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Down, Over, Up

If you start at the North Pole and walk one mile south, one mile east, and one mile north, you’ll find yourself back at your starting point.

The North Pole is not the only point with this property on Earth’s surface. In fact, there are any number of such points. Where are they?

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“The Four Sevens”

Another puzzle from Henry Ernest Dudeney:

http://www.gutenberg.org/files/16713/16713-h/16713-h.htm

“In the illustration Professor Rackbrane is seen demonstrating one of the little posers with which he is accustomed to entertain his class. He believes that by taking his pupils off the beaten tracks he is the better able to secure their attention, and to induce original and ingenious methods of thought. He has, it will be seen, just shown how four 5’s may be written with simple arithmetical signs so as to represent 100. Every juvenile reader will see at a glance that his example is quite correct. Now, what he wants you to do is this: Arrange four 7’s (neither more nor less) with arithmetical signs so that they shall represent 100. If he had said we were to use four 9’s we might at once have written 99 9/9, but the four 7’s call for rather more ingenuity. Can you discover the little trick?”

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Source Forge

http://en.wikipedia.org/wiki/Image:SupperatEmmaus-Meegeren.jpg

One of these Vermeers is a forgery. Which is it?

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The Bridges of Konigsberg

In old Konigsberg there were seven bridges:

http://commons.wikimedia.org/wiki/Image:Konigsberg_bridges.png
Image: Wikimedia Commons

Villagers used to wonder: Is it possible to leave your door, walk through the town, and return home having crossed each bridge exactly once?

Swiss mathematician Leonhard Euler had to invent graph theory to answer the question rigorously, but there’s a fairly intuitive informal proof. Can you find it?

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