Question: why 99 is not a prime number?
Answer: For 99, the answer is: No, 99 is not a prime number. The list of all positive divisors (i.e., the list of all integers that divide 99) is as follows: 1, 3, 9, 11, 33, 99. For 99 to be a prime number, it would have been required that 99 has only two divisors, i.e., itself and 1.