To add the fractions \( \frac{3}{10} \) and \( \frac{2}{5} \), we first need to have a common denominator.
The denominators are 10 and 5. The least common multiple of 10 and 5 is 10.
Now, we can convert \( \frac{2}{5} \) to a fraction with a denominator of 10:
\[ \frac{2}{5} = \frac{2 \times 2}{5 \times 2} = \frac{4}{10} \]
Now we can add the two fractions:
\[ \frac{3}{10} + \frac{4}{10} = \frac{3 + 4}{10} = \frac{7}{10} \]
So, \( \frac{3}{10} + \frac{2}{5} = \frac{7}{10} \).