Calculate the Sum: 20.4 + 10.4 Decimal Addition Problem

To solve this problem, we'll add the decimal numbers $20.4$ and $10.4$ by aligning the decimal points:

• Step 1: Write the numbers in a column aligning their decimal points:

$\begin{array}{r} 20.4 \\ +10.4 \\ \hline \end{array}$

• Step 2: Add the numbers starting from the right. First, $4 + 4 = 8$.
• Next, add the digits to the left of the decimal point: $0 + 0 = 0$.
• Finally, add the leftmost digits: $2 + 1 = 3$.

Combining these, the result is $30.8$.

Thus, the solution to the problem is $\mathbf{30.8}$.