A small stone is inside a ice block which floats in water. When the ice fully melts the level water

$$\begin{gathered} \mathrm{Before} \mathrm{melting}, \mathrm{weight} \mathrm{of} \mathrm{combined} \mathrm{mass} \mathrm{is} \mathrm{equal} \mathrm{to} \\ \mathrm{weight} \mathrm{of} \mathrm{the} \mathrm{liquid} \mathrm{displaced}. \\ \left({\mathrm{m}}_1+{\mathrm{m}}_2\right)\mathrm{g}={\mathrm{V}}_{\mathrm{i}}{\mathrm{\rho }}_{\mathrm{w}}\mathrm{g} \end{gathered}$$.

$\mathrm{where} {\mathrm{V}}_{\mathrm{i}}=\mathrm{volume} \mathrm{of} \mathrm{ice} \mathrm{and} \mathrm{stone} \mathrm{system} \mathrm{inside} \mathrm{the} \mathrm{water}.$

$$\begin{gathered} \therefore {\mathrm{V}}_{\mathrm{i}}=\frac{{\mathrm{m}}_1+{\mathrm{m}}_2}{{\mathrm{\rho }}_{\mathrm{w}}} \\ \Rightarrow {\mathrm{V}}_{\mathrm{i}}=\frac{{\mathrm{m}}_1}{{\mathrm{\rho }}_{\mathrm{w}}}+\frac{{\mathrm{m}}_2}{{\mathrm{\rho }}_{\mathrm{w}}}.............\left(1\right) \end{gathered}$$

$$\begin{gathered} \mathrm{After} \mathrm{the} \mathrm{ice} \mathrm{is} \mathrm{melted} {\mathrm{m}}_1 \mathrm{converts} \mathrm{into} \mathrm{water} \mathrm{and} {\mathrm{m}}_{2 }\mathrm{remains} \mathrm{same}. \\ \mathrm{and} \mathrm{completely} \mathrm{submerged} \mathrm{in} \mathrm{water}. \end{gathered}$$

$$\begin{gathered} \mathrm{Volume} \mathrm{of} \mathrm{water} \mathrm{formed} \mathrm{by} {\mathrm{m}}_1(\mathrm{mass} \mathrm{of} \mathrm{ice}) \\ {\mathrm{V}}_1=\frac{{\mathrm{m}}_1}{{\mathrm{\rho }}_{\mathrm{w}}} \end{gathered}$$

$$\begin{gathered} \mathrm{Volume} \mathrm{of} \mathrm{stone} \mathrm{is} \mathrm{equal} \mathrm{to} \mathrm{volume} \mathrm{ofwaater} \mathrm{displced} \\ {\mathrm{V}}_{\mathrm{st}}=\frac{{\mathrm{m}}_2}{{\mathrm{\rho }}_{\mathrm{s}}} \end{gathered}$$

$$\begin{gathered} \mathrm{Total} \mathrm{volume} \mathrm{after} \mathrm{the} \mathrm{ice} \mathrm{melts} \\ \mathrm{V}=\frac{{\mathrm{m}}_1}{{\mathrm{\rho }}_{\mathrm{w}}}+\frac{{\mathrm{m}}_2}{{\mathrm{\rho }}_{\mathrm{st}}}...........\left(2\right) \end{gathered}$$

$\mathrm{since} {\mathrm{\rho }}_{\mathrm{st}}>{\mathrm{\rho }}_{\mathrm{w}}\Rightarrow \mathrm{V}<{\mathrm{V}}_{\mathrm{i}}$

$\mathrm{equation} \left(1\right) \mathrm{and} \left(2\right)\Rightarrow \mathrm{level} \mathrm{of} \mathrm{water} \mathrm{decreases}.$