Calculate the mass percent (w/w) of sulphuric acid in a solution prepared by dissolving 4g of sulphur trioxide in a 100ml of sulphuric acid solution containing 80 mass percent (w/w) of ${\mathrm{H}}_2{\mathrm{SO}}_4$ and heving a density of $1.96\mathrm{g}/\mathrm{ml}$.

(Molecular weight of ${\mathrm{H}}_2{\mathrm{SO}}_4=98\mathrm{gm}$)

Take reaction ${\mathrm{SO}}_3+{\mathrm{H}}_2\mathrm{O}⟶{\mathrm{H}}_2{\mathrm{SO}}_4$