Three bodies having masses 5 kg, 4 kg and 2 kg are moving at the speeds of $5 {\mathrm{ms}}^{-1}, 4 {\mathrm{ms}}^{-1}$$\mathrm{and}$$2 {\mathrm{ms}}^{-1},$ respectively along X-axis. The magnitude of velocity of centre of mass nearly is 

Velocity of centre of mass,

          ${\nu }_{\mathrm{CM}}=\frac{m_1{v}_1+m_2{v}_2+m_{3}3}{m_1+m_2+m_3}$

Given, ${\mathrm{m}}_1=5 \mathrm{kg}, {\mathrm{v}}_1=5{\mathrm{ms}}^{-1},$

           ${\mathrm{m}}_2=4 \mathrm{kg}, {\mathrm{v}}_2=4 {\mathrm{ms}}^{-1}, {\mathrm{m}}_3 = 2\mathrm{kg} \mathrm{and} {\mathrm{v}}_3 = 2 {\mathrm{ms}}^{-1}$

Substituting all these values in above equation, we get

           ${\nu }_{\mathrm{CM}}=\frac{5\times 5+4\times 4+2\times 2}{5+4+2}=\frac{25+16+4}{11}$

$\therefore v_{\mathrm{CM}}=\frac{45}{11}=4.09\approx 4{ \mathrm{ms}}^{-1}$