Two spherical bodies of mass M and 5M and radii R and 2 R are released in free space with initial separation between their centres equal to 12 R. If they attract each other due to gravitational force only, then the distance covered by the smaller body before collision is : 

7.5 R

Initial distance between their centers = 12 R

At time of collision the distance between their centers = 3 R

So total distance travelled by both=12 R – 3 R = 9 R

Since the bodies move under mutual forces, center of mass will remain stationary so

\(m_1x_1 = m_2x_2\)

\(m_1 = m ; m_2 = 5m ; x_1 = x ; x_2 = 9 R - x \)

⇒ x = 7.5 R