When $2 \mathrm{~g}$ of a gaseous substance $\mathrm{A}$ is introduced into an initially evacuated flask at $25^{\circ} \mathrm{C}$, the pressure is found to be $1 \mathrm{~atm} .3 \mathrm{~g}$ of another gaseous substance B is added to it at the same temperature and pressure. The final pressure is found to be $1.5 \mathrm{~atm}$. Assuming ideal gas behaviour, the ratio of molar masses of A and B is :