Sunday, March 25, 2007

Puzzling Week: Getting to 100

From Car Talk Puzzler

RAY: This is mathematical in nature. I want you to get a pencil and write down the numbers, 1 to 9, inclusive, and leave enough space between them. At your disposal you have one plus sign and two minus signs. You can insert those plus and minus signs wherever you want, to make the total come out to 100. In other words, if you were to put an equal sign after the 9, you want to be able to write 100 after it.

Now I'll give you an example, you could do 12 + 345 - 67 - 89. It doesn't work, but you get the idea. You can't change the order. You can't monkey with the plus or minus signs.

You've got one plus sign and two minus signs, you can put them any place you want, and the total has to equal 100. What's the equation?

What strategies did you use to solve this?



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