In a cubic close packed lattice of oxide ions there would be two tetrahedral voids and one octahedral void for each oxide ion.
∴ For four oxide ions there would be 8 tetrahedral and four octahedral voids out of which one tetrahedral void is occupied by A and two octahedral voids are occupied by B.
% of tetrahedral voids occupied by A = 1/8 x 100 = 12.5%
% of octahedral voids occupied by B = 2/4 x 100 = 50%