[richardkiss]

The following theorem, originally imparted to me by an algebra professor in the UCLA math department (grad school), may be helpful to you when testing yourself:

Theorem. All numbers < 100 that look prime, are prime EXCEPT 91.

(My addendum)

Where "look prime" means, is not a multiple of 2, 3, 5, 11, nor a perfect square. All these numbers have quick tests for divisibility.

Note that 91 = 7 * 13.

[sanderjd]

That's awesome. Came here to say that 91 keeps killing my gut-instincts!