**Prime numbers**History / Mystery / Mathematics / Cryptography

## Definition

An integer number is a *multiple of 3* if it is **divisible by 3**.

### A mathematical trick to determine whether a number is divisible by 3

In order to easily find out whether a number is divisible by 3, we sum its digits, and study whether this sum is equal to 0, 3, 6 or 9, in which case our number is indeed divisible by 3. If the sum is larger than or equal to 10, we again sum up the digits of the sum, and so on and so forth, until we get a number smaller than 10.

For example, let us study whether 2 027 is a multiple of 3.
The sum of its digits is 2+0+2+7=11; this number is larger than 10, so we sum up again.
1+1=2, which is different from 0, 3, 6 or 9, and therefore *2 027 is not divisible by 3*.

### A few suggestions

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- Is 2 a prime number?
- Is 55 a prime number?
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