Decimal Addition: Solving 0.4 + 0.4

To solve this problem, we'll follow these steps:

• Step 1: Align the decimal points vertically in the numbers to be added.
• Step 2: Add the numbers as if they were whole numbers, placing the decimal correctly in the sum.

Let's perform the addition:

Step 1: Align the numbers $0.4$ and $0.4$: $\begin{array}{c} \phantom{+}0.4 \\ + 0.4 \\ \hline \end{array}$

Step 2: Add $4$ and $4$ in the tenths place, which gives us $8$. The decimal point remains in the same position, resulting in $0.8$.

Therefore, the sum of $0.4 + 0.4$ is $0.8$.