$m_A=400g$
حجم آب بیرون ریخته:$100cm^3 \Rightarrow V_A=100cm^3$
$\Rightarrow \rho = \frac {m_A}{V_A} = \frac {400g}{100cm^3}=4 \frac {g}{cm^3}$
$\left. \begin{matrix}
{{\rho }_{A}}=2{{\rho }_{B}} \\
{{\rho }_{A}}=4\frac{g}{c{{m}^{3}}} \\
\end{matrix} \right\}\Rightarrow {{\rho }_{B}}=2\frac{g}{c{{m}^{3}}}$
$\left. \begin{matrix}
{{V}_{B}}=50c{{m}^{3}} \\
{{\rho }_{B}}=2\frac{g}{c{{m}^{3}}} \\
\end{matrix} \right\}\,,\,{{\rho }_{B}}=\frac{{{m}_{B}}}{{{V}_{B}}}\Rightarrow 2\frac{g}{c{{m}^{3}}}=\frac{{{m}_{B}}}{50c{{m}^{3}}}\Rightarrow {{m}_{B}}=100g$