PV = nRT
PV = $\frac{w}{M} RT$
V = $\frac{w}{M} \, \frac{RT}{P}$
Here R, T and P are constants
$\therefore $ $V_{He} \, \propto \frac{w(He)}{M(He)}$
and $V_{CH_4} \, \propto \frac{w(CH_4)}{M(CH_4)}$
But $V_{He}= V_{CH_4} $ (Given)
$\frac{w(He)}{M(He)} = \frac{w(CH_4)}{M(CH_4)}$
$\frac{w(He)}{4} = \frac{w(CH_4)}{16}
$ \frac{w(He)}{M(CH_4)} = \frac{1}{4}$
$w(CH_4)$ = 4 w (He)$
Wt.% of $CH_4 $ = $\frac{w(CH_4) \times 100}{w(CH_4) + w(He)}$
= $\frac{4 \times 100}{5} $ = 80%