what is the greatest common factor of the numbers 12 and 16

If you break a number down to it's prime factorization you can always determine the GCF.

What that means is you write the number as the product of only prime numbers.

12 = 2 * 2 * 3

16 = 2 * 2 * 2 * 2

Remember the C in GCF means Common

What do 12 and 16 have in common?

2 * 2 or 4