According to Raoult's law,
$\large\frac{P^0-P_s}{P^0}=$$n_B=\large\frac{n_B}{n_A+n_B}$
$\Rightarrow \large\frac{n_B}{n_A}$
$\Rightarrow \large\frac{W_B\times M_A}{M_B\times W_A}$
(For dilute solutions,$n_B < < < n_A$)
$M_B=\large\frac{W_B\times M_A}{W_A}.\frac{P^0}{P^0-P_s}$
$\Rightarrow \large\frac{0.2\times 18}{99.8}\times \frac{1.013}{1.013-1.009}$
$\Rightarrow 4.06g mol^{-1}$