I just realized this, so I’m putting it here before I forget it :-) When looking at factors for any number using the Excel spreadsheet format, you only need to do the math for a number (X) between 1 and the square root of X in order to determine if there are any factors greater than the square root of X. Thus, to find whether X is prime, you only need to test up to the square root of X, and if you find no factors then X is prime.
