Step 1: Know the difference between rate of reaction and rate of change of A.
For $2A \rightarrow$ product, two molecules of A are used up for the reaction to happen once. So the rate of reaction is the rate at which A falls, divided by 2.
Step 2: Write the formula.
\[ \text{Rate} = -\frac{1}{2}\frac{\Delta[A]}{\Delta t} \]
Step 3: Find how much A dropped.
A went from 0.5 M down to 0.4 M, a fall of $0.1$ M over 10 minutes.
Step 4: Put the values in.
Rate $= \dfrac{1}{2} \times \dfrac{0.1}{10} = \dfrac{0.1}{20} = 0.005$ mol $L^{-1}$ min$^{-1}$.
Step 5: Write in scientific form.
$0.005 = 5 \times 10^{-3}$ mol $L^{-1}$ min$^{-1}$.
Step 6: Conclusion.
So the answer is $5 \times 10^{-3}$ mol $L^{-1}$ min$^{-1}$. \[ \boxed{5 \times 10^{-3}} \]