Solid AB has NaCl structure Atom A occupy the corners of the cubic unit cell. If all the face centered atoms along one of the axes are removed then the total number of atoms of A & B is  equal to _______

According to the structure of NaCl 

Here, ′A′ will occupy 8 corners of cubic unit cell which will contribute 1/8​ each to the cube therefore 8×1/8​=1

and 6 face centered atoms will contribute1/2​ to the cube.
Hence 6×1/2​=3

Therefore, total contribution of atom'A' is 3+1=4
Now B occupies octahedral voids (NaCl structure) which contributes 1/4
therefore 1+12×1/4​=4
So, the formula is A is-
A₄​B_4​or AB
Now we are removing all face centered atoms along one axis-
so the number of atoms removed from cube are 2
so, contribution from 'A' remains 8×1/8​(corner atoms) +4×1/2​(facecenteredatoms)=3
so the resultant becomesA₃​B_4​