Two particles of masses 4 kg and 6 kg are at rest separated by 20 m. If they move towards each other under mutual force of attraction, the position of the point where they meet is

Since the blocks are moving under mutual attraction, their centre of mass will not move at all. 

So eventually the blocks should meet at their centre of mass to keep their centre of mass constant. 

Assuming 4kg blocks is at x = 0, 

${\mathrm{X}}_{\mathrm{CM}}= \frac{{\mathrm{m}}_1{\mathrm{x}}_1+{\mathrm{m}}_2{\mathrm{x}}_2}{{\mathrm{m}}_1+{\mathrm{m}}_2}$

$= \frac{4\times 0+6\times 20}{4+6}=12\mathrm{m}$

Hence the correct answer is 12m from 4kg body.