is 20 a prime number yes or no?
For 20, the answer is: No, 20 is not a prime number. The list of all positive divisors (i.e., the list of all integers that divide 20) is as follows: 1, 2, 4, 5, 10, 20. For 20 to be a prime number, it would have been required that 20 has only two divisors, i.e., itself and 1.