$200\mathrm{mL}\text{ of water is added to }500\mathrm{mL}\text{ of }0.2\mathrm{M}\text{ solution. What is the molarity of this diluted solution? }$

$$\begin{gathered} \text{ No. of millimoles }=500\times 0.2=100 \\ \text{ Thus, molarity of diluted solution }=\frac{100}{700} \\ \text{ (Molarity }=\text{ No. of moles }{\mathrm{L}}^{-1}=\text{ No. of millimoles }{\mathrm{mL}}^{-1}\text{ ) }=0.1428\mathrm{M} \end{gathered}$$