Solve the Decimal Equation: Finding the Missing Value in 0.5 - ? = 0.2

To solve the problem $0.5 - ? = 0.2$, let's proceed with the following steps:

• Step 1: Begin with the equation: $0.5 - ? = 0.2$.
• Step 2: To isolate '?', add 0.2 to both sides of the equation. This gives us:

$0.5 - ? + 0.2 = 0.2 + 0.2$.

• Step 3: Simplify the equation:

$0.5 = ? + 0.2$.

• Step 4: Isolate '?' by subtracting 0.2 from 0.5:

$? = 0.5 - 0.2$.

• Step 5: Perform the subtraction:

$? = 0.3$.

Therefore, the solution to the problem is $\boxed{0.3}$. This matches with choice 3 of the given options.