16
1 and 4
The factor of what.
12, 16 and 18
5, 10, 15 and 20 have five as a factor. The number 16 has five factors.
12, 16 or 18
1, 2, 4, 5, 10, 20, 25
The prime numbers between 1 and 20 are: 2, 3, 5, 7, 11, 13, 17 and 19. Note that a prime number has only two factors, 1 and itself and one. Number 1 is not considered to be a prime number because it has only one factor.
The only common factor is 1.
The GCF is 1.
The two number are coprime: their common factor is 1.
1 and 5
1 and 5