Mole fraction of A in ${\mathrm{H}}_2\mathrm{O}$ is 0.2. The molality of A in ${\mathrm{H}}_2\mathrm{O}$ is :

molality = moles of solute/ solvent's wt. (kg)

$\begin{array}{l}=\frac{0.2}{(0.8\times 18)}=\frac{1}{4\times 18}\times 1000\mathrm{mol}/\mathrm{kg}\\ =0.0139\times 1000\\ =13.9\mathrm{mol}/\mathrm{kg}\end{array}$