Three identical spheres each of radius R are placed such that their centres lie on a straight line. What is the location of their centre of mass from the centre of the first sphere ?

2R

Distance between first and last sphere = R + 2R + R = 4R

Since the spheres are identical and lie in a straight line, the centre of mass will lie exactly in the middle.

Hence the centre of mass lies at a distance of 2R from the centre of the first sphere.