First discard all the evens, they can't be prime. Then those divisible by 5.

That leaves 211 which may be prime. Now check it it is divisible by 3, 7, 11, 13 if it isn't then it is prime (you check its divisibility by all primes less than or equal to its square root, but you have already dealt with 2 and 5 so no need to do those again)

The result that we are using is that if a number is composite then it has a divisor less than (or equal to) its square root.

CB