[tex]C_p=\frac{m_s}{m_r}\cdot100\%\\\\V_r=500 \ cm^3\\\\d_r=1,2 \ \frac{g}{cm^3}\\\\m_r=V_r\cdot d_r=500 \ cm^3\cdot1,2 \ \frac{g}{cm^3}=600 \ g\\\\\frac{m_s}{600g}\cdot100\%=20\% \ \ |:100\%\\\\\frac{m_s}{600g}=0,2 \ \ |\cdot600 \ g\\\\\huge\boxed{m_s=120 \ g}[/tex]
