Solve the Decimal Equation: Finding the Missing Addend in 0.4 + __ = 0.51

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

• Step 1: Identify the given information
• Step 2: Use subtraction to find the missing value
• Step 3: Verify the correct answer among the choice options

Now, let's work through each step:
Step 1: The problem gives us $0.4 + \_\_\_ = 0.51$. We need to find the missing number.
Step 2: To find the missing number, subtract $0.4$ from $0.51$:
$0.51 - 0.4 = 0.11$

Step 3: Verify this result matches one of the provided answer choices.
The missing number is $0.11$, which corresponds to choice $0.11$.

Therefore, the solution to the problem is $0.11$.