How to Do a Simple Number Mind Trick

Simple Number Prediction

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.

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.

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.

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.

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.

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.

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.

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.

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.

Guessing Someone's Age

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.

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.

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.

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.

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.

Ask for his last answer. Have him say the final number out loud, so you and everyone in the room can hear it.

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.

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). Impress friends at a party. "I was looking for a way to break the ice at a small get-together. This simple number prediction trick was a big hit! I had everyone amazed that I could read their minds. It sparked fun conversations trying to guess how it worked." - Syeda S. Bond with grandkids over math magic. "My grandkids visit every summer and I'm always looking for new activities. The age guessing trick was perfect for making math entertaining. They begged me to teach them after I guessed their ages exactly right." - Phoebe M. Liven up math lessons with tricks. "As a 5th grade teacher, I'm always trying to make math exciting. These number tricks have been a great way to engage my students. They get a kick out of trying to stump each other with different variations. Their math skills improve while having fun!" - Susan H. Impress a date with mentalist abilities. "I wanted to surprise my date with something unique on our next outing. This article gave me some great math-based tricks to try out. When I guessed her age exactly, she was extremely impressed! It led to a fun conversation about math and magic." - Jithin K. Did you know that wikiHow has collected over 365,000 reader stories since it started in 2005? We’d love to hear from you! Share your story here.

Magical 37

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.

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.

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.

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,

Announce that his number is 37. As long as your volunteer followed the instructions correctly, his answer will always be 37.