William Wallace sent me the following email so I'm inspired to promote his puzzle:
I came across your math blog, and thought you might be interested in a puzzle I came up with.
To see how it works, go to (http://blog.coincidencetheories.com/?p=1522). You probably can figure it out first, then go verify. You can blog on it. I released it to the public domain so you don't need to give credit, but you may. Leave a pingback--I honor them. The above website has a pdf version.
How to play:
In order to calculate the number your child picked, you must be able to add two digit numbers in your head. If you can do this, think about the final sum, and start off by setting it to 0 in your mind. As you show each page, if your child picks a blue(cross) or green(square), find the lowest value on the page that is blue(cross), or green(square), whichever she picked, and add that to your sum. If your child picked red(circle), simply add zero (0) to your sum. Repeat this for each of the next three pages. After the forth page, the sum in your mind is the number that your child picked. Announce it. Your child will think you're a mind reader.