20 ml of pure acetic acid (density $=0.75 \mathrm{g} {\mathrm{mm}}^{-1}$ ) is mixed with 50 gm of water (density $=1 \mathrm{g} \mathrm{m}{\mathrm{ml}}^{-1}$ ) at a certain temperature. Calculate the molality of acetic acid in the final solution.