Eight solid uniform cubes of edge $l$ are stacked together to form a single cube with center $\mathrm{O}$. One cube is removed from this system. Distance of the centre of mass of remaining $7$ cubes from $\mathrm{O}$ is

Initially one edge of all 8 cubes will be at origin 

If we consider the position of C.O.M of the 7 cubes at a distance of ‘x’  from origin, and centre of mass of a single cube will be at a distance of $\frac{l\sqrt{3}}{2}$ from edge(origin).

Considering $\mathrm{O}$ as origin, the center of mass of the stacked system(8 cubes) will be at origin.

$\frac{7mx+ {\displaystyle \frac{ml\sqrt{3}}{2}}}{8m}=0 \Rightarrow x=-\frac{\sqrt{3}l}{14}$

So the center of mass of the 7 cubes will be at a distance of $\frac{\sqrt{3}l}{14}$from the origin.