### Theory:

1. Addition of rational numbers with the same denominator.
When we have the same denominators, all we have to do is add the numerators and write the answer with the resultant numerator and same denominator.
Example:
$\frac{2}{7}+\frac{3}{7}=\frac{5}{7}$
2. Addition of rational numbers with different denominators.
We have to convert the different denominators to the same denominator. To do that, take the LCM of the denominator, then multiply both rational numbers by the LCM so that we get the equivalent rational numbers with the same denominator.
Example:
Add the two rational numbers $\frac{2}{5}$ and $\frac{3}{4}$.

To the given rational numbers, we can use the above theory.

$\begin{array}{l}\frac{2}{5}+\frac{3}{4}=\frac{\left(2×4\right)+\left(3×5\right)}{20}\\ \\ =\frac{8+15}{20}\\ \\ =\frac{23}{20}\end{array}$

Therefore, the sum of $\frac{2}{5}$ and $\frac{3}{4}$ $$=$$ $\frac{23}{20}$.