A solid is made of two elements P and Q. Atoms P are in ccp arrangement and atoms Q occupy all the octahedral voids and half of the tetrahedral voids, then the simplest formula of the compound is

Four atoms (P) contribute to one unit cell from ccp arrangement and 4-atoms (Q) from the all octahedral voids and 4-atoms (Q) from the half of the tetrahedral void contributes one unit cell. So, formula of solid is P4Q8, so, the simplest formula of the solid is PQ2                                                          OR In ccp lattice points are 4 . P=4Octahedral voids are 4 and tetrahedral voids are 8.Q occupy all octahedral voids(4) and half of the tetrahedral voids .(8×12=4)Thus formula P4Q8 OR PQ2