## Mathematical cloning brainteaser

By David Shaw, 30 September 2020

**Difficulty: Taxing**

Grab a calculator (or a pen and paper)

Choose a three-digit number (e.g. 123)

Multiply it by 7

Multiply the answer by 11

Multiply that answer by 13

You will get your starting number written out twice! (e.g. 123 123)

The question is, can you work out why?

**Scroll down or click for a hint, or the answer!**

## Brainteaser hint

What is 7 x 11 x 13? You can use a calculator to help you, if you need it!

## Brainteaser answer

Multiplying by 7, 11 and 13 will give you the same answer as if you multiplied by 1001.

7 x 11 x 13 = 1001

And 1001 = 1000 + 1

So if your starting number is 123:

123 x 7 x 11 x 13

= 123 x 1001

= 123 x 1000 + 123 x 1

= 123 000 + 123

= 123 123

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