Solve the Decimal Equation: Finding the Missing Number in 0.1 + ? = 0.4

To solve this problem, we need to find what number, when added to $0.1$, results in $0.4$. We can solve this using simple arithmetic operations as follows:

• Step 1: Understand that the equation provided is $0.1 + ? = 0.4$.
• Step 2: To find the missing value $?$, we need to rearrange the equation by isolating $?$ on one side: $? = 0.4 - 0.1$.
• Step 3: Conduct the subtraction: $0.4 - 0.1 = 0.3$.

Therefore, the number that needs to be added to $0.1$ to equal $0.4$ is $0.3$.

Reviewing the provided multiple-choice options, the correct choice is the third option: $0.3$.