If you avoid certain obvious non-prime numbers, all two digit numbers are prime numbers except for 49 and 91. Why? Because all other numbers either are even or mulitples of 5.
Don't belive it? Give it a try.
Similarly after avoinding certain obvious non-prime numbers, you are left with just 75 three-digit numbers that are divisible by 7, 13, 17, 19, 23, 29, or 31.
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