Particles of masses 2kg  and 4kg  are at  (i+j+k)  m and (−i+j−k) m then instantaneous position of their centre of mass is

Sol:- rCM=m1r1+m2r2m1+m2                                =2(i+j+k)+4(−i+j−k)6                                =2i+2j+2k−4i+4j−4k6                              =16[−2i+6j−2k]