In a cubic structure of compound which is made from X and Y, where X atoms are at the corners of the cube and Y at the face centres of the cube. The molecular formula of the compound is:

X atoms are at eight corners of the cube. Therefore, the number of X atoms in the unit cell $=\frac{8}{8}=1$. Y atoms are at the face centre of six faces. Therefore, its share in the unit cell $=\frac{6}{2}=3.$ The formula is XY₃.