I know how much money you are carrying – why the math trick works

I don’t usually like generic email forwards, but I make an exception for ones that involve math tricks.

A couple years ago I got an email about how a waiter could calculate your age using “restaurant math.” It was an interesting trick and I posted about how the it worked: see “your age by diner and restaurant math” explanation.

Today I came across another interesting math trick. The idea is that I can figure out how much money you are carrying by doing some simple arithmetic.

Give it a try to see if it works for you.

The trick

Here’s one of those fun math tricks I got in the email.

Don’t tell me how much money you have; I do not wish to be nosy, but I bet I can figure it out.

It takes less than a minute. Work this out as you read. Be sure you don’t read the bottom until you’ve worked it out!

This is not one of those waste of time things, it’s fun.

1. First of all, count how much money you have in your wallet. If you have small change, round to the nearest dollar (if you have 55.15 dollars, then just round to 55 dollars.)

2. Multiply this number by 2 (just to be bold)

3. Add 3 to the result

4. Multiply by 5 (if needed, use a calculator to figure out)

5. Now subtract 6 from what you get

You will end up with a number that ends in the digit 9. Remove this digit and keep the remaining number (ex. 459 —> 45)

The rest of the numbers will equal THE MONEY IN YOUR WALLET!

If you like this trick, please forward to your friends!

(email adapted from this handout)

Example of the process

Let’s say you started with $123. Let’s see how each step would work.

First you multiply this number by 2 —> 246

Then you add 3 to the result —> 249

Next you multiply by 5 —> 1245

Finally you subtract 6 —> 1239.

The last digit is 9 and the remaining digits are 123–exactly how much money you started with!

That’s pretty cool. So why does this math trick work?

Explanation of trick

Of course there no such thing as math magic–it can always be explained by the formulas.

I came up with a small diagram to explain exactly why this trick works.

Here is the reason:

(click here or click picture for bigger image of derivation)