$m=\rho .V\Rightarrow 6{{\rho }_{A}}=8\times 0/9\Rightarrow {{\rho }_{A}}=1/2\frac{g}{c{{m}^{3}}}=1200\frac{kg}{{{m}^{3}}}$ $V=\frac{m}{\rho }\Rightarrow \frac{300}{{{\rho }_{A}}}=\frac{600}{{{\rho }_{B}}}\Rightarrow {{\rho }_{B}}=2{{\rho }_{A}}=2/4\frac{g}{c{{m}^{3}}}=2400\frac{kg}{{{m}^{3}}}$ $\rho =\frac{{{m}_{A}}+{{m}_{B}}}{{{V}_{A}}+{{V}_{B}}}=\frac{10+30}{\frac{10}{1200}+\frac{30}{2400}}=\frac{40\times 2400}{50}=1920\frac{kg}{{{m}^{3}}}$ صفحۀ ۲۲ فيزيک ۱