A B=B C=C D=D A=20 cm Since, all spheres are identical with mass 1kg and radius 10cm each, hence according to figure, it is clear that all spheres are symmetrically arranged, hence centre of mass will be at intersection point of diagonals of square A B C D. ∴‌AC=√202+202=20√2cm ∴‌AO=BO=CO=DO=‌