IIT JEE Advanced 2012 Paper 1

As X is present at the corners of the cube and also at the faces of the cube, so the contribution of X will be
8 (corners) ×

1

8

× (contribution of each corner atom) + 6 (faces) ×

1

2

(contribution of each face) = 4
As M is present at the body centre and at each diagonal, so its contribution will be
4 (diagonals) ×

1

4

(contribution of each diagonal) + 1 (body centre) = 2
So, the unit cell formula becomes M2X4 and the empirical formula will be MX2.