2 Math Magic Tricks: Guess Someone’s 3-Digit Number or Age

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Everyone knows magic tricks are fun, but not enough people realize that math can be as well. [1] Whether you want to teach students or just have fun with your friends, we've found 2 impressive tricks that will give them good surprise. Just make sure they don't find this page before you!

Method 1
Method 1 of 2:

The 1089 Trick

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  1. This trick only involves addition and subtraction, but some people get confused by the instructions. It works best on a friend who will pay close attention, and who is unlikely to make a math mistake.
  2. Announce that you'll write down a "magic number" on a piece of paper. Write down 1089 without showing anyone, then fold the paper in half.
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  3. Tell him not to show you the number or tell you what it is. Make sure he understands that none of the three digits can be the same.
    • For example, he could pick 481 .
    • He might need a simple calculator as well.
  4. On the next line underneath the number, he should write down the same digits in reverse order.
    • For example, 481 backwards is 184 .
  5. Now that your volunteer has two numbers, have him subtract the smaller one from the larger one.
    • 481 - 184 = 297 .
  6. Now ask him if his number is two or three digits long, without telling you the actual number. If it is only two digits long, have him place a 0 at the beginning.
    • In our example, 297 has three digits, so you can skip this step. Sometimes your friend will end up at 99, and this step will turn that into "099."
  7. Ask him to take his answer and reverse the digit order again. If he added the zero the beginning, remind him to include it as part of the number.
    • For example, the reverse of 297 is 792 .
  8. As the final calculation, your friend adds the last two numbers he wrote down.
    • In our example, 792 + 297 = 1089 .
  9. Announce that you know the last number he wrote down. Unfold the piece of paper and reveal the 1089 you wrote down earlier.
    • The answer is always 1089. If your friend has a different answer, he did not follow instructions, or he made a mistake.
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Method 2
Method 2 of 2:

Guessing Age or Shoe Size

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  1. 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!
  2. 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.
  3. 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 .
  4. 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.
  5. The volunteer should multiply her answer by two. (This is where a calculator comes in handy.)
    • 2115 x 2 = 4230 .
  6. 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 .
  7. 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 .
  8. 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. [2]
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  • Question
    When using method number two, what should I do if the first number is greater than the last?
    Community Answer
    Regardless of which number is first, the guide instructs you to subtract the smaller number from the larger one.
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  • Question
    What is the difference between distance and displacement?
    Community Answer
    Displacement is the shortest distance between two points whereas distance is the length between two points.
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  • Question
    What do I do if someone doesn't know the shoe size when doing the math magic trick?
    Community Answer
    If they don’t know their shoe size, then make them pick any shoe size. For example, if they pick shoe size 1, at the end you would tell them they picked shoe size 1.
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      Tips

      • The 1089 trick actually works with most 3-digit numbers, even if two of the digits are repeated. It does not work with numbers that are the same backwards as forwards (such as 161 or 282). Asking for three different digits is an easy way to avoid this. [3]
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        Helpful 2 Not Helpful 0
      • Do not repeat the trick in front of the same person! If you do so, he/she could easily figure the trick out and purposely ruin the trick the next time you do it in front of someone else and, make it look like you were the one who flopped. It can be really embarrassing, especially when you are doing it before a crowd or at a party.
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      • Don't repeat a trick around the same group of people. It's much less impressive when you predict 1089 the second time, for example!
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        Helpful 11 Not Helpful 4

      Tips from our Readers

      The advice in this section is based on the lived experiences of wikiHow readers like you. If you have a helpful tip you’d like to share on wikiHow, please submit it in the field below.
      • Practice these tricks on yourself a few times to be sure you're doing it right. Then, show them to other people.
      • The 1089 trick does not work if the participant gives a mirrored number (for example, 313 or 595 or 676).
      • Don't perform the same trick for the same person multiple times or else they'll figure out the trick.
      • Be very clear and don't rush through things when you're doing the trick or people may get confused.
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      References

      1. ↑ https://www.umassmed.edu/bsrc/tricks/
      2. ↑ https://www.realclearscience.com/blog/2014/10/the_math_behind_the_shoe_size-age_trick_108920.html
      3. ↑ https://www.mathsisfun.com/1089.html
      4. Age guessing trick adapted from https://nrich.maths.org/1051

      About This Article

      Article Summary X

      If you want to amaze your friend with a math magic trick, start by asking them to write down their age on a piece of paper and tell them not to show you anything they write. Then, have them multiply their age by 5 and add a 0 to the number they get. Once they have that number, tell them to add today’s date. For example, if it’s august 9th, they would add 9 to their current number. Once they add the date, ask them to multiply the number by 2 before adding their shoe size. Make sure they round their shoe size up to the nearest whole number since you don’t want any decimals. Finally, have them subtract today’s date twice to reveal the final number, the first part of which should be their age and the last 2 digits their shoe size. For more tips, like how to do the 1089 trick, read on!

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