All but 56789 are.
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three and 17
3 x17 is 51 51 is not a prime number it is composite. use this rule : You wil know a number is divisible by 3 if you add up the digits. if the sum is a multiple of 3 then that number is divisible by 3. i.e.: 51 5+1 =6 6 is divisible by 3 so 51 is divisible by 3. 51/3= 17
1, as are all numbers. oh, and 17.
1, 3, 7, 17, 21, 51, 119, 357
3, 5, 15, 17, 25, 51, 75, 85, 255, 425.