Two bottles of sodium chloride solution are poured together: 200 mL at 0.5 mol/L and 300 mL at 0.2…

Step 1. Moles, in millimoles. $0.5 \times 200 = 100$ and $0.2 \times 300 = 60$.

Step 2. Total. $160$ mmol in $500\ \text{mL}$.

Step 3. Molarity. $\dfrac{160}{500} = 0.32$ mol/L. In code: return (M1 * V1 + M2 * V2) / (V1 + V2).