16
1 and 5
The difference is between factor pairs and distinct factors. With square numbers, one of the factor pairs will be the same number twice. When listing the distinct factors, that number is only listed once.
The factors of 4 are 1, 2, and 4. The only prime factor of 4 is 2. The factors of 20 are 1, 2, 4, 5, 10, and 20. The prime factors of 20 are 2x2x5.
20 has six factors.
30 is the only number between 20 and 50 that is a factor of 60
For a number to have [only] 2 factors, it must be a prime number; the prime numbers between 0 and 20 are; 2, 3, 5, 7, 11, 13, 17, 19
28
20 and 28 both have six factors.
The factors of 16 are 1, 2, 4, 8, and 16.
Because they only have two factors, as do 17 and 19.
48
12, 18 and 20 each have six factors.
16 has five factors.
16 has five factors.
5, 10, 15 and 20 have five as a factor. The number 16 has five factors.
All of them. 2 3 5 7 11 13 17 19 have only two factors.