It's the Amazing Math Trick!

Some friends were over for pizza the other day, and one of the kids (an eighth grader) said she wanted to show me a math trick. I said sure, and so she asked me to pick any number. I mentally chose my favorite number, but then she could somehow tell that I had made an unusual choice. So, she told me to pick a whole number. I accepted her new constraint and said I had picked a new number. Again she was still suspicious, so she insisted that I pick a positive whole number. Deflated, I chose accordingly and followed along as she had me:

1. double it,
2. add 6,
3. divide by 2, and then
4. subtract the original number

She magically told me the result of my calculations with the number I never shared with her. Only a moment later did I realize that I should have stuck with my original number (e is about 2.718281828459045…, but it never stops or repeats). Doing the process with the symbol e instead of the whole number I chose (13) would have yielded the same result and would have illustrated the “math trick” at play.

Math tricks are fun, but getting kids to expand their ideas about the power of mathematical reasoning is even more fun.

Related Links

• Teach Mama: Mind Blowing Math Tricks
• Math Tricks
• thinktanK12 blog: Math