Let the number of atoms of B in the ccp structure be $N$. The number of tetrahedral voids is $2N$. Atoms of A occupy 1/3 of the tetrahedral voids, so the number of atoms of A is $\frac{1}{3} \times 2N = \frac{2}{3}N$. The ratio of A:B is $\frac{2}{3}N : N = 2:3$. Thus, the formula is $A_2B_3$. Here $x=2$ and $y=3$, so $x+y = 2+3 = 5$.