A compound is formed by elements A and B. The crystalline cubic structure has the A atoms at the corners of the cube and B atoms at the body centre. The simplest formula of the compound is

Atoms are present in the corners of cube = A
Atoms present at body centre = B.
We know that a cubic unit cell has 8 corners. Therefore contribution of each atom A at the corner = $\frac{1}{8}$. Since number of atoms per unit cell is 8, therefore total contribution = $8\times \frac{1}{8}=1$. We also know that atoms in the body centre, therefore nurnber of atoms of B per unit cell = 1. Thus formula of the compound is AB.