[tex]\boxed{\rho= \frac{m}{V} } \\ \rho_1=1200 \ kg/m^3 \\ \rho_2=800 \ kg/m^3 \\ m_1=m_2=m_3 \\ \rho_3= \frac{m_3}{V_3} \\ V_3=V_1+V_2 \\ V_1= \frac{m_1}{\rho_1} , \ iar \ V_2= \frac{m_2} {\rho_2} \\ \Rightarrow \rho_3= \frac{m_3}{ \frac{m_1}{\rho_1} + \frac{m_2}{\rho_2} } =\frac{2 \cdot \rho_1 \cdot \rho_2}{\rho_1+\rho_2} \\ \^{I}nlocuim: \\ \rho_3= \frac{2 \cdot 1200 \ kg/m^3 \cdot 800 \ kg/m^3}{1200 \ kg/m^3+800 \ kg/m^3} = \frac{2 \cdot 960000}{2000 \ kg/m^3} = \frac{1920000}{2000 \ kg/m^3} \\ [/tex]
[tex]\Rightarrow \boxed{\rho_3=960 \ kg/m^3}.[/tex]
