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 = 8
Contribution to each unit cell =  $\frac{1}{8}$
Effective number of atoms =  $8 \mathrm{x}\frac{1}{8}=1$
Atoms ‘B’ 
Number of face centres = 6
Contribution to unit cell =  $\frac{1}{2}$
Since   one atom of a  face  centre is missing
Effective atoms of ‘B’ =  $5 \mathrm{x} \frac{1}{2}=\frac{5}{2}$
Formula of the compound   ${\mathrm{AB}}_{\frac{5}{2}}\left(\mathrm{or}\right){\mathrm{A}}_2{\mathrm{B}}_5$