Question

Is 23 prime or composite?

Answer 1

It's prime

Explanation:

You may try and divide by any number, there will always be a leftover, called the modus.
23/2=11, 1 left, or 23mod2=1
23/3=7, 2 left, or 23mod3=3
(don't try 4, it would already be divisable by 2)
23/5=4, 3 left, or 23mod5=3

In fact you won't have to try all the way up to 23, or even 5.