A compound formed by elements X and Y has a cubic structure in which X atoms are at the corner of the cube and also at alternate face centres. Y atoms are present at the body centre and also at  the alternate edge centre of the cube. The value of Z_eff and formula of the compound are respectively.

X atoms are at the corners of the cube. There are 8 corners in a cube.

X atoms are also at alternate face centres. There are 6 faces in a cube.

Y atoms are at the body centre and at alternate edge centres of the cube. 

There is 1 body centre and 8 edges in a cube.

So, the contribution of X atoms per unit cell = $\frac{1}{8}\times 8+2\times \frac{1}{2}=2$

So, the contribution of Y atoms per unit cell = $1+4\times \frac{1}{4}=2$

So, the ration of X atoms and Y atoms = 2:2 = 1:1

The formula of the compound = XY

So the Z_eff (total number of X and Y atoms contributing per unit cell)= 2 + 2 = 4.