How to Do a Number Trick to Guess Someone's Age

There are lots of number tricks you can do by using math. You can give someone a particular set of instructions and calculations and their current age can be deduced. To them it appears as though you are performing a magic trick, but all you need to know are the instructions. The math will work every time. Another set of instructions can be given to get the month and day of their birth. You can also use math to appear to guess the age of a stranger.

Method 1
Method 1 of 3:

Deducing Someone’s Age Using a Selected Number

  1. Image titled Do a Number Trick to Guess Someone's Age Step 1
    1
    Ask your friend to choose a number between two and ten. To make the game more fun, you might ask them how many times per week they would like to have ice cream, go out to eat, or something similar. Once they have a number in mind, ask them to announce it.[1]
    • Say that they choose the number 6. This will be the example used throughout this method.
  2. Image titled Do a Number Trick to Guess Someone's Age Step 2
    2
    Multiply the given number by two. You can do this math by hand or your friend can use a calculator for the rest of the trick. Explain to your friend that they must hit the equal (=) button after each instruction you give.
    • Example: 6 x 2 = 12.
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  3. Image titled Do a Number Trick to Guess Someone's Age Step 3
    3
    Add five to the answer.
    • Example: 12 + 5 = 17.
  4. Image titled Do a Number Trick to Guess Someone's Age Step 4
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    Multiply that number by 50.
    • Example: 17 x 50 = 850.
  5. Image titled Do a Number Trick to Guess Someone's Age Step 5
    5
    Ask your friend if they’ve already celebrated their birthday for this year. If their birthday has passed already, then tell them to add 1767 to the prior result. If their birthday is coming up, then ask them to add 1766 to the prior result.
    • Example 1: (with prior birthday): 850 + 1767 = 2617.
    • Example 2: (with upcoming birthday): 850 + 1766 = 2616.
    • Be aware that the numbers are for the year 2017. You need to add a number for each year that passes. For 2022 it is 1771 and 1772 if they have had their birthday.
  6. Image titled Do a Number Trick to Guess Someone's Age Step 6
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    Subtract the year that they were born.
    • Example 1: 2617 - 1981 (year of birth) = 636
    • Example 2: 2616 – 1981 (year of birth) = 635
  7. Image titled Do a Number Trick to Guess Someone's Age Step 7
    7
    Look at your final answer. The first digit, the one in the hundred’s place, will be the original one chosen. The remaining two numbers will reveal your friend’s age.
    • Example 1: The answer is 636. 6 was the original number chosen. 36 is the current age.
    • Example 2: The answer is 635. 6 was the original number chosen. 35 is the current age.
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Method 2
Method 2 of 3:

Using a Calculator to Find Someone’s Age

  1. Image titled Do a Number Trick to Guess Someone's Age Step 8
    1
    Ask the person to multiply the first number of their age by five. For the sake of this example, let’s say the person is 35 years old. They can use a calculator or scratch paper for this. Let the person know that they must hit the equal (=) button after each instruction you give.[2]
    • Example: 5 x 3 = 15.
  2. Image titled Do a Number Trick to Guess Someone's Age Step 9
    2
    Tell them to add three.
    • Example: 15 + 3 = 18.
  3. Image titled Do a Number Trick to Guess Someone's Age Step 10
    3
    Tell them to double the resulting figure.
    • Example: 18 x 2 = 36.
  4. Image titled Do a Number Trick to Guess Someone's Age Step 11
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    Have them add the second number of their age to that number.
    • Example: 36 + 5 = 41.
  5. Image titled Do a Number Trick to Guess Someone's Age Step 12
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    Tell them to subtract six. The answer will be their current age.[3]
    • Example: 41 – 6 = 35.
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Method 3
Method 3 of 3:

Using a Calculator to Deduce the Month and Date of Birth

  1. Image titled Do a Number Trick to Guess Someone's Age Step 13
    1
    Enter the number seven. Then, multiply it by the month of their birth. For this example, let’s say their date of birth is May 28, 1981.[4]
    • Example: 7 x 5 (birth month: May) = 35.
  2. Image titled Do a Number Trick to Guess Someone's Age Step 14
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    Subtract one. Then, multiply by 13.[5]
    • Example: 35 – 1 = 34
    • Then: 34 x 13 = 442.
  3. Image titled Do a Number Trick to Guess Someone's Age Step 15
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    Add the day of their birth.[6]
    • Example: 442 + 28 = 470.
  4. Image titled Do a Number Trick to Guess Someone's Age Step 16
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    Add three. Then, multiply by 11.[7]
    • Example: 470 + 3 = 473.
    • Then: 473 x 11 = 5,203.
  5. Image titled Do a Number Trick to Guess Someone's Age Step 17
    5
    Subtract the month of their birth. Then, subtract the day of their birth.[8]
    • Example: 5,203 - 5 (May) = 5,198.
    • Then: 5,198 – 28 = 5,170.
  6. Image titled Do a Number Trick to Guess Someone's Age Step 18
    6
    Divide by 10. Then, add 11.[9]
    • Example: 5,170 ÷ 10 = 517.
    • Then: 517 + 11 = 528.
  7. Image titled Do a Number Trick to Guess Someone's Age Step 19
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    Divide by 100. The first digit, before the decimal, is their birth month (May). The digits after the decimal denote the day of birth (28).[10]
    • Example: 528 ÷ 100 = 5.28.
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Community Q&A

  • Question
    This doesn't make sense. How does 17x50 equal 850? I thought it was 400.
    Community Answer
    Community Answer
    Check your multiplication again. 17x50 does equal 850.
  • Question
    Could I also subtract their birth year from the current year to determine their age?
    Alexis Styles-Rainies-Thompson
    Alexis Styles-Rainies-Thompson
    Community Answer
    No. It’s either current year if their birthday has passed, or last year if their birthday hasn’t passed yet.
  • Question
    Is this just a trick or does it really help you guess someone's age?
    Community Answer
    Community Answer
    If done correctly, this should actually help you determine a person's age.
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Warnings

  • Make sure to take your time with the math or you may miss a step and end up with an incorrect result.
    ⧼thumbs_response⧽
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References

  1. ↑ https://www.engadget.com/2013/11/12/use-your-iphone-to-impress-your-friends-with-these-cool-calculat/
  2. ↑ http://cybraryman.com/numbertricks.html
  3. ↑ http://www.learn-with-math-games.com/math-number-tricks.html
  4. ↑ http://www.dr-mikes-math-games-for-kids.com/magical-calculator-birthday-math-trick.html
  5. ↑ http://www.dr-mikes-math-games-for-kids.com/magical-calculator-birthday-math-trick.html
  6. ↑ http://www.dr-mikes-math-games-for-kids.com/magical-calculator-birthday-math-trick.html
  7. ↑ http://www.dr-mikes-math-games-for-kids.com/magical-calculator-birthday-math-trick.html
  8. ↑ http://www.dr-mikes-math-games-for-kids.com/magical-calculator-birthday-math-trick.html
  9. ↑ http://www.dr-mikes-math-games-for-kids.com/magical-calculator-birthday-math-trick.html
  1. ↑ http://www.dr-mikes-math-games-for-kids.com/magical-calculator-birthday-math-trick.html

About This Article

wikiHow Staff
Co-authored by:
wikiHow Staff
wikiHow Staff Writer
This article was co-authored by wikiHow Staff. Our trained team of editors and researchers validate articles for accuracy and comprehensiveness. wikiHow's Content Management Team carefully monitors the work from our editorial staff to ensure that each article is backed by trusted research and meets our high quality standards. This article has been viewed 468,210 times.
181 votes - 46%
Co-authors: 24
Updated: July 19, 2022
Views: 468,210
Categories: Magic Tricks
Article SummaryX

To do a number trick to guess someone’s age, start by asking them to use a calculator to multiply the first number of their age by 5. Then, have them add 3 before doubling the answer. Next, they should add the second number of their age. Finally, tell the person to subtract 6 and the answer will be their current age. For tips on how to do a number trick to guess someone’s age without a calculator, read on!

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