Atoms ‘A’ occupy corners of cube and atoms ‘B’ occupy face centres. If one atom of a face centre is missing then the formula of the compound would be

Atoms ‘A’Number of corners = 8Contribution to each unit cell =  18Effective number of atoms =  8 x18=1Atoms ‘B’ Number of face centres = 6Contribution to unit cell =  12Since   one atom of a  face  centre is missingEffective atoms of ‘B’ =  5 x 12=52Formula of the compound   AB52 (or) A2B5