Okay, so I've thought about it and now I think I know what the question means.
GCF of 2 numbers means that 2 numbers share the same Greatest Common Factor. That said, since we know the greatest common factor, we need to multiply 9 to get those 2 numbers.
9 X 1 = 9
9 X 2 = 18
9 X 3 = 27
9 X 4 = 36
9 X 5 = 45
These are the only multiples you need to figure out the rest.
9 X 1 is unnecessary when we look at multiples and factors because it is just the same number to finish.
9 X 4 isn't helpful because large even numbers can just be divided by 2 to get the GCF. (It isn't 9)
9 X 5 can be divided by 15, which makes 15 the GCF rather than 9.
So that leaves us with 9 X 2 and 9 X 3. But we're not done yet.
Why must both numbers be a multiple of 9?
Simply because in order to get to the numbers in the first place, you have to multiply by nine. So a way to check your work for this is to multiply 9 X 2, and then divide by 9 again to make sure that it is a real number. That sounds really complicated, but it's beyond simple when you look at it like this:
9 X 2 = 18
18 / 9 = 2
And there ya go! Hope this helps.
