Particles of masses 1 kg and 3 kg are at (2i + 5j + 13k) m and (-6i + 4j - 2k) m. What is the position of their centre of mass?

1/4 (-6i + 17j + 5k) m

Sum of masses = 1 + 3 = 4 

x-coordinate : (1 x 2-3 x 6)/4 = -16/4

y-coordinate : (1 x 5 + 3 x 4)/4 = 17/4

z-coordinate : (1 x 13 - 3 x 2)/4 = 7/4

Position of centre of mass = 1/4 (-6i + 17j + 5k) m.