A present in corners and face centre.
B present in alternate tetrahedral voids.
Along one body diagonal, two atoms of A are present and one atom of B is present. After removal of all the atoms along the body diagonal, number of A per unit cell =6×18+6×12=154
Number of B per unit cell =4−1=3
x:y=154:3=5:4
x+y=5+4=9