Question

State whether the number 91 is prime, composite, or neither.

Answer 1

To test any number N for primality, you only need to test all primes from 2, 3, 5,... until square-root of N.

So for 91, you need to test all primes as factors up to 9, namely 2,3,5,7.
Try that and post what you find.

Answer 2

I think you will understand much if you think about why the square root thing is true.