Is the number 57 prime or composite?

Answer 1

You can see quickly that the number 57 is divisible by 3:

3 * 20 = 60 so if we remove one of those 3's then

3 * 19 = 57 so it is divisible by at least 3 and 19 (as well as itself and 1)

Because it is divisible by numbers other than itself and one, 57 cannot be prime.

You can also check to see if a number is divisible by 3 by adding its digits and checking to see if that number is divisible by 3:

5+7 = 12 ; and 12 is divisible by 3, so 57 is as well.