All of them.
2 3 5 7 11 13 17 19 have only two factors.
28
2,3,5,7,11,13,17,19
20 and 28 both have six factors.
The factors of 16 are 1, 2, 4, 8, and 16.
48
Because they only have two factors, as do 17 and 19.
12, 18 and 20 each have six factors.
16 has five factors.
16 has five factors.
2, 3, 5, 7, 11, 13, 17, 19
5, 10, 15 and 20 have five as a factor. The number 16 has five factors.
2, 3, 5, 7, 11, 13, 17, and 19 each have two factors . . . '1' and itself.