The velocities of three particles of masses 10 kg

The velocities of three particles of masses 10 kg, 20 kg and 30 kg are 10i, 10j and 10k m/s, respectively. What is the velocity of their centre of mass?
A. (i + 2j + 3k) m/s
B. 10 (i + j + k) m/s
C. (10i + 20j + 30k) m/s
D. (3i + 2j + k) m/s

The correct choice is A. (i + 2j + 3k) m/s

The velocity of the centre of mass = (m1*v1 + m2*v2 + m3*v3)/(m1 + m2 + m3)
 = (10*10i + 20*10j + 30*10k) / (10 + 20 + 30)
 = (100i + 200j + 300k) / 100
 = (i + 2j + 3k) m/s.