American in Spain

Brainteasers: Missing Things

April 29, 2011

Missing SquareToday I wanted to share two of my favorite brainteasers. It's just by chance that they are so similar, both tricking you into thinking something is missing. Every few years, I forget the trick to the first one – because I'm smart like that – and it stumps me again. The latter puzzle is more of an optical illusion than anything else. Like all good optical illusions, understanding how it works doesn't ruin your ability to see it.

The Missing Dollar

Three men go to a restaurant for dinner and spend $25. Each man gives the waiter $10. The waiter keeps $2 as a tip and gives $1 back to each man. Thus each man pays $9, and so the group pay $9 x 3 = $27. The waiter keeps $2, making a total of $29. Where did the missing dollar go?

The Missing Square

Both triangles are made up of the same pieces, but one has a missing square. Why?

Missing Square

If you're stumped, click on the image to go to Wikipedia for the mathematical solution, or watch this excellent video that my friend, Ray, made, which demonstrates the solution.