Since the gases react at a distance 1.5m from the lighter end, and the time taken should be the same,

$\Rightarrow$ Time $=\large\frac{0.5 \times 3}{r_{\text{lighter gas}}}$$=\large\frac{0.5 }{r_{\text{heavier gas}}}$

$\Rightarrow \large\frac{3}{\sqrt M_{\text{lighter gas}}} $$ = \large\frac{1}{\sqrt M_{heavier gas}} $$\rightarrow M_{\text_{Lighter gas}}: M_{\text_{Heavier Gas}} = $$ \large\frac{1}{3^2} $$= \large\frac{1}{9}$