-11
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87 is the only 2-digit number that is 6 greater and 13 less than a square, but it is not prime.
13
13 13 x 2 = 26 13 x 6 = 78 13 x 4 = 52
Prime numbers less than 18 are: 2 3 5 7 11 13 and 17
28 is greater than 13 2/8 is less than 1/3