When you see a fraction, you see a numerator and a denominator. To find the common factor, see whichever number among these is least: The numerator, the denominator or the difference between numerator and denominator.
Learning multiplication facts is an essential part of childhood math. It takes time to learn them, but with a few tips and tricks, you can conquer them with practice. Once you have them memorized, you can work on getting faster with them so that the numbers just come to you. Some of the multiplication facts are simpler than others, but many of them have helpful hints that make them easier to learn.