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.
Division with remainders. Your child will most likely come across or ask about situations where division “does not work.” These can be explained with the introduction of the remainder. It is an important idea to understand as the division of larger numbers will require the “carrying” of this remainder.