To find out if a number is a prime number, you can divide it by two. If it is a whole number, it isn't prime. It would be a mixed number/fraction if so.
<em>All prime numbers are whole numbers but the result of the division shouldn't be when you divide the number you want to find out. If it is a fraction/mixed number, your unknown number is most likely a prime number. (my explanation was confusing)</em>