The atomic weight of two elements A and B are 40 and 80 respectively. If  $x$  g of A contains y atoms, how many atoms are present in  $2x$g of B ?

                                         A                    B
Atomic mass                 40                 80
Given weight                  x gram          2x gram
No. of moles                  x/40              2x/80
No. of atoms             $\frac{x}{40}$ x $N_A$              $\frac{x}{40}$ x $N_A$               
But according to question =   $\frac{x}{40}$ x $N_A$ = y