is created by David Witten, a mathematics and computer science student at Vanderbilt University. For more information, see the "About" page.

Train Fly Riddle

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The Riddle

Two trains are 100 miles apart. They travel toward each other at 20 miles/hour. One fly leaves off the front of the train at 40 miles/hour and flies until it hits the other train. It turns around and comes back. It keeps doing this until the trains collide. How far does the fly travel?


Note: One way to solve this problem is to calculate how far the fly flies each time before it hits the other train. From that you can find an infinite series. However, this is a lot of work and is not worth it. Alternatively, you should figure out the amount of time the fly flies. It takes 2.5 hours for the trains to collide. The fly always flies at 40 mph, so it flies 100 miles.



Boy/Girl Population