Two particles of masses 1g and 2g move towards each other with velocities $10m{s}^{-1}$ and $20m{s}^{-1}$ respectively. The velocity of the center of mass of the system of the two particles is

Given, 
Particle one, mass  $\left(m_1\right)$=1 g
Particle two, mass  $\left(m_2\right)$=2 g
Velocity of first particle $\left(v_1\right)=10m/s$  
Velocity of second particle $\left(v_2\right)=20m/s$
Velocities are in opposite direction
Velocity of center of mass is given by
 $v_{cm}=\frac{m_1{v}_1+m_2{v}_2}{m_1+m_2}=\frac{1\times 10+2(-20)}{1+2}=-\frac{30}{3}=-10m/s$
Hence, magnitude of velocity of center of mass is 10 m/s.