Well, we would have to start solving this by dividing. (I used my calculator for this one, because the outcome was not going to be a whole number.).
So 64 / 7, we get 9.14285714286.
Okay, imagine you have let's say, 10, 7 oz. cups. We are trying to fill them all to the brim. Now, we can't have that extra left over decimal, it would overflow! So the best bet would be to jot this 9.14, down to a 9. In the end, we can fill 9/10 cups.