Sometimes when we add fractions, the sum can be reduced even though we have used the least common denominator. Other times, the sum cannot be reduced. For example, if we add 1/3 + 1/6 The sum 3/6 can be reduced to 1/2. However, 215 + 1/6 = 17/30 cannot be reduced ,Let's look at another example:
4/21 +7/15= 20/105+49/105=69/105
This can be reduced to 23/105. However, when we add 2/21 + 4/15 we obtain 10/105 + 28/105 = 38/10. which cannot be reduced. There are pair. of denominators for which it seems we never can reduce the sum. for
example, 7/10+11/12 =42/60+55/60=97/60