The atomic masses of two elements A and B are 20 and 40 respectively. If x gm of A contains Y atoms, how many atoms are present in 2x gm of B?

Formula that shows the relation between the moles and number of atoms.

$\mathrm{Mole}\times {\mathrm{N}}_{\mathrm{a}}=\mathrm{Number} \mathrm{of} \mathrm{atoms}$

Here, N_a is Avogadro's number.

Apply the given conditions:

$\mathrm{x}\times \frac{{\mathrm{N}}_{\mathrm{a}}}{20}=\mathrm{y}$

For 2x g, 

$\begin{array}{ccc}\frac{2}{2}\left(\mathrm{x}\times \frac{{\mathrm{N}}_{\mathrm{a}}}{20}\right)& =& \mathrm{y}\\ 2\mathrm{x}\times \frac{{\mathrm{N}}_{\mathrm{a}}}{40}& =& \mathrm{y}\end{array}$

Above expression states that 2x g of B contains Y atoms.