Particles of masses 12 kg and 6 kg are released from a separation 60 m and move towards each other under the mutual gravitational force. they will hit each other at a distance of: 

Since the bodies are moving due to mutual gravitational force and there is no external force, they collide at their centre of mass.

${\mathrm{X}}_{\mathrm{cm}}=\frac{{\mathrm{m}}_1{\mathrm{x}}_1+{\mathrm{m}}_2{\mathrm{x}}_2}{{\mathrm{m}}_1+{\mathrm{m}}_2}=\frac{12\times 0+6\times 60}{18}=20\mathrm{m}$

20m from 12kg body's initial position.