To find the common denominator of fractions, the LCM process is used.
To illustrate;
[tex]\frac{1}{3}+\frac{1}{5}[/tex]
Note that the denominators of both fractions are different, that is, 3 and 5.
To solve this kind of math problem, the first step is to take a common denominator for both fractions. In this example, a common multiple for 3 and 5 would now be 15 (that is, the least of ALL possible multiples of 3 and 5)
Hence, the next step would be to apply the LCM to the denominators of both fractions, as follows;
[tex]\begin{gathered} \frac{1}{3}+\frac{1}{5} \\ \text{now becomes,} \\ \frac{5}{15}+\frac{3}{15} \\ \text{And finally,} \\ \frac{8}{15} \end{gathered}[/tex]
The word to fill into the gap in your question is COMMON.
You need to find the "common" denominator
