How to Do a Math Magic Trick

Everyone knows magic tricks are fun, but not enough people realize that math can be as well. Whether you're teaching students or just having fun with your friends, these tricks will give them a good surprise.

Guessing Age and Shoe Size

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   Ask a volunteer to write down her age. Give her a piece of paper and instruct her not to show you what she's writing.

   • This trick won't work on someone 100 years old or above, but that's almost never a problem!
   • Have her multiply it by 5. Ask her to follow your instructions as you tell her what calculations to solve. Start by asking her to multiply her age by 5.

     • For example, if someone is 42 years old, she would write down 42 x 5 = 210.
     • Let her use a calculator if she'd prefer to.
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     Write a zero at the end of the answer. This is the same as multiplying by 10, but phrasing it this way makes it more difficult for the volunteer to follow the trick.

     • In our example, adding a zero to 210 makes 2100.
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     Add today's date. It doesn't actually matter what she adds here — we'll undo it later — but today's date is an easy small number to add. Mention the date aloud to make sure she knows it.

     • For example, if today is March 15th, the volunteer in our example would sum 2100 + 15 = 2115.
     • Tell her to ignore the month and year.
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     5
     Double the answer. The volunteer should multiply her answer by two. (This is where a calculator comes in handy.)

     • 2115 x 2 = 4230.
     • Add the volunteer's shoe size. Ask the volunteer to write down her shoe size, and to round up if it's not a whole number. She should add this to her last answer.

       • If her shoe size is 7, she'll add 4230 + 7 = 4237.
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       Subtract twice today's date. It's best to calculate this in your head, then tell her to subtract the number you come up with.

       • For example, it's March 15th in our example, so multiply 15 x 2 = 30 in your head. Say "Subtract 30 from your answer" and the volunteer will calculate 4237 - 30 = 4207.
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       8
       Reveal the magic. Tell her to read her answer out loud. The first part of the number is her age, and the last two digits are her shoe size.