# Why isn't 57 a prime number?

Apr 6, 2016

$57$ is not a prime number, as it has factors other than one and itself.

#### Explanation:

A prime number is one who does not have any factor other than one and itself.

$57$ has $1$ and $57$ as its factor, but it also has $3$ and $19$ as its factor.

Hence, $57$ is not a prime number.

May 10, 2017

$57$ is divisible by $3$

$57 = 3 \times 19$

#### Explanation:

At first glance $57$ would seem to be a prime number ....

after all, $7 , \text{ "17," "37," "47," "67," } 97$ are all prime.

Why not 57?

If you add the digits of $57$ you get $5 + 7 = 12$

$12$ is a multiple of $3$
This means that $57$ is also a multiple of $3$

The nearest multiples of $3$ to $57$ are:

$60$ - which is $3 \times 20$ and $54$ - which is $3 \times 18$

$57$ is $3$ away from both of these and is actually $3 \times 19$

There are $25$ prime numbers from 1 to 100, but there are $5$ numbers which warrant extra attention, because they look as though they might be prime but are actually not. Learn them!

These are " "color(red)(1," "51," "57," "87," "91)

$1$ has only one factor, not two factors as prime numbers have.

$51 , 57 \mathmr{and} 87$ are all multiples of $3$. (add their digits)

$91 = 7 \times 13$

As it is a product of two primes and above the normal times tables, we will not often have come across $91$ in our maths.