The GMAT only expects one to know primes less than 100. If they are asking you to test for primes greater than 100, then it will be likely a different concept. For example, is 11! + 7 prime and why or why not?
But coming back to the primes less than 100, I prefer not to tax my brain by memorizing the numbers, instead only look for odd numbers(other than 2 which is prime) and see if they are divisible by 3, 5, and 7, if they are not divisible by 3, 5, and 7, then it is prime.
For example, 89 is not divisible by 3(digits add up to 17), is not divisible by 5(does not end in 0 or 5), and also not divisible by 7(you do have to actually do the division but it should be fairly quick). Therefore, 89 is prime.
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