NaCl has a face centered cubic close packing in which lattice points are occupied by Cl- ions whereas Na+ ions occupy all octahedral holes.Here Na+ & Cl- both have a coordination number of 6.

  

   Effective number of A- or Cl-  = (8*1/8)+(6*1/2) = 4

   

   Effective no. of A- after removing atoms along one axes

                = (8*1/8)+(4*1/2) = 3

 

   Effective no. of B+ or Na+ = (12*1/4) + 1 = 4

 

               

             Therefore the formula is A3B4