How to Do a Simple Number Mind Trick

Last Updated: July 8, 2022

Amaze your friends and relatives with these number-based mind tricks. These three tricks are arranged from easiest to hardest (smallest numbers to biggest). Even young kids can do the simple number prediction trick.^{[1]
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Method 1

Method 1 of 3:

Simple Number Prediction

1
Set the trick up. Tell a friend that you are performing a math trick. You will ask her to do some calculations secretly, then read her mind for their answer.
2
Write down your prediction. Pretend to think hard for a moment, then write down the number 3 on a piece of paper. Fold the paper in half, without letting anyone see the number.

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3
Ask your friend to write down a number between 1 and 20. She should pick this secretly and keep the piece of paper.
- We'll go through an example, where your friend secretly picks the number 4.
- This trick works with any number, but keeping it between 1 and 20 makes it less likely that your friend will make a mistake.
4
Instruct her to add 1 to her number. Warn her not to give you any sign, not even moving her lips. All you need is your mental powers.
- For example, if she chose 4, her new number is 4 + 1 = 5.
5
Tell your friend to double the new number. Have her take the last number she had, and multiply it by 2.
- 5 x 2 = 10.
6
Have her add 4 more. Put your hands to your head and concentrate, then tell her to add 4 to her last answer.
- 10 + 4 = 14.
7
Divide by 2. Tell her you almost have it, but the number is too big for you to see. Ask her to divide it by 2 to make it easier.
- 14 ÷ 2 = 7.
8
Subtract the original number. Ask your friend to look at the piece of paper she used to remind herself what the original number was. Take the last answer she had and subtract that original number.
- 7 - 4 = 3.
9
Show her your prediction. Tell her you've finally read her mind. Ask her to announce the last number she ended up with. Once she has, unfold your piece of paper and reveal the 3 you wrote down. No matter which number she started with, her answer will be 3.

Method 2

Method 2 of 3:

Guessing Someone's Age

1
Tell someone you're going to find his age. Let him know you'll be using your mathematical mind-reading abilities. Give him a calculator if he doesn't want to do the calculations in his head.
- This trick won't be too impressive with close friends or classmates, since you probably already know their age.
- Pick someone at least 10 years old and no more than 99 years old.
2
Ask him to multiply the first digit in his age by five. Remind him to do the math silently, to keep his age secret from you.
- For example, if he is 32 years old, he should take the 3 and multiply it by 5. The answer is 3 x 5 = 15.
3
Add 4 to his answer. Tell him to add 4 to the answer he found.
- In our example, he would silently add 15 + 4 = 19.
4
Tell him to double his answer. Now your subject should multiply his last answer by 2, and let you know when he's finished. If he's doing the math in his head, ask him "Are you sure?" since people are most likely to make a mistake on this step.
- 19 x 2 = 38.
5
Have him add the second digit of his age. Next, the person you're "mind reading" adds the last number in his age. Tell him this is the last calculation he needs to do.
- Since the subject of our example is 32 years old, he would add 2 to his last answer. His last answer was 38, so he calculates 38 + 2 = 40.
6
Ask for his last answer. Have him say the final number out loud, so you and everyone in the room can hear it.
7
Subtract 8 and tell him his real age. Silently subtract 8 from the number he gave you. The answer is his age, which you should announce to the room.
- In our example, 40 - 8 = 32.
8
Try some variations. If you perform this trick more than once, people might figure out how it works. Here are a couple variations to keep it mysterious:
- Instead of adding 4 and later (secretly) subtracting 8, you can add 3 and subtract 6, or add 2 and subtract 4, or even add 25 and subtract 50. Just remember you'll need to subtract twice as much as the original number, because it got doubled in one of the later steps.
- To really switch it up, try this: double your age, add 2, multiply by 5, and subtract 10. You need both the doubling and multiplication by five to move the first digit of his age (3 in our example) to the tens place where it belongs (3 x 2 x 5 = 30).

Method 3

Method 3 of 3:

Magical 37

1
Hand a volunteer a pencil and paper. This one involves some three digit numbers, so most people will not want to do the math in their head. Warn the volunteer that he will need to do long division.
2
Ask him to write the same digit 3 times. Tell him to hide the paper from you to prove you're not cheating. Ask him to write a 3-digit number that repeats the same digit three times.
- For example, he could write 222.
3
Instruct him to add each digit together. Now your volunteer should treat each of the three digits as a separate number, and find their sum.
- For example, 2 + 2 + 2 = 6.
4
Tell him to divide the larger number by the smaller one. He should now take the 3-digit number, and divide it by the\ smaller one. Give him some time to work this out.
- 222 / 6 = 37,
5
Announce that his number is 37. As long as your volunteer followed the instructions correctly, his answer will always be 37.^{[2]
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Community Q&A

Question

What do you do if someone won't tell you the first digit of their age?

Living Concrete

Top Answerer

Well, that means they don't want to play the game. If they refuse to play, just leave him or her alone.
Question

How does the third trick work?

Living Concrete

Top Answerer

The third trick works because 111 divided by 3 is 37. So long as 111 and 3 are multiplied by the same multiple, the answer will always be 37. For example, if 111 (the dividend) is multiplied by 2, 3 (the divisor) should be multiplied by 2 as well in order to keep 37 as the quotient.
Question

Why does the first one never work? I did it perfectly! Ugh.

Community Answer

Maybe the person you did it with made a few mistakes? Mistakes are okay, try it again.