Twelve charges have been placed at the centre of each side of a cube as shown in the figure. The magnitude of electric force acting on a charge Q placed at the centre of the cube is  $\frac{yKQq}{r^2}$. Then value of “y” is:
Where r is side of the cube.

Consider 4 charges on the square ABCD. Force due to these 4 charges
$F_1=2\sqrt{2}\frac{kQq}{r^2}$  parallel to BC
Similarly, force due to 4 changes on square EFGH is 
$F_2=2\sqrt{2}\frac{kQq}{r^2}$  parallel to GH
Resultant of   ${\overrightarrow{F}}_{1}and{\overrightarrow{F}}_2$  will be  $\frac{4kQq}{r^2}$ parallel to PQ.
 
Force due to 4 charges at I,J,K,L is also parallel to PQ and has magnitude $F=\frac{4KQq}{r^2}$ 
Hence, resultant is  $\frac{8KQq}{r^2}$ along PQ