If you cannot spot a common denominator, you can find one by multiplying the denominators of the fractions you are trying to add or subtract.
(You will be shown how to find the numerators for the equivalent fractions later).
This method is quite easy to use, when there are only a few fractions with fairly small denominators. For more than three fractions, or for larger denominators, the common denominator produced by the multiplication tends to get quite large. This makes the work of finding the numerators for the equivalent fractions and reducing the final answer to its lowest form, more difficult.