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How to Do the Calculator Prediction Trick

Your friends will think you can read minds when you use only a calculator to guess their secret number.


  • Step 1: Have friend write a number Tell your friend to write any number on a sheet of paper. The number can be as many digits long as they want.
  • TIP: Have each person in a group contribute one digit to form a final number that you can use in the trick.
  • Step 2: Write the number nine Tell your friend you will predict the final number they will have at the end of the trick. Write the number nine on another sheet of paper, fold the paper, and set it aside.
  • Step 3: Scramble the digits Have your friend scramble the digits of their number and write it down. If the original number is 435, they might write down 354.
  • TIP: Have your friend write the larger of the two numbers above the other number to avoid getting a negative total when you subtract.
  • Step 4: Subtract Have your friend use the calculator to subtract one number from the other.
  • Step 5: Add Have your friend add the digits of the number together until only one digit is left.
  • TIP: Be sure that your friend uses the calculator when doing the math calculations. This trick won't work if the math is wrong.
  • Step 6: Show your prediction Show your friend your prediction on the piece of paper. Ask, "Is this your final number?" and prepare to watch their jaws drop in amazement at your magic skills.
  • FACT: A Greek mathematician in the second century invented the basis of modern algebra when he began using symbols for representation in math problems.

You Will Need

  • A friend
  • Paper
  • A pen
  • A calculator

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