Three identical spheres, each of mass \( 1 \mathrm{~kg} \) are plac...

Three identical spheres, each of mass \( 1 \mathrm{~kg} \) are placed touching each other with their centres on a straight line. Their centre are marked \( \mathrm{P}, \mathrm{Q} \) and \( \mathrm{R} \) respectively. The distance of centre of mass of the system from \( P \) is :
(1) \( \frac{\mathrm{PQ}+\mathrm{PR}+\mathrm{QR}}{3} \)
(2) \( \frac{P Q+P R}{3} \)
(3) \( \frac{\mathrm{PQ}+\mathrm{QR}}{3} \)
(4) \( \frac{\mathrm{PR}+\mathrm{QR}}{2} \)