At $600 \mathrm{~K}$, the root mean square (rms) speed of gas $\mathrm{X}$ (molar mass $=40$ ) is equal to the most probable speed of gas $\mathrm{Y}$ at $90 \mathrm{~K}$. The molar mass of the gas $\mathrm{Y}$ is ___________ $\mathrm{g} ~\mathrm{mol}^{-1}$. (Nearest integer)