Explanation

(a)
$Number\ of\ moles\ of\ He = \frac{0.4}{4} = 0.1$
$Number\ of\ moles\ of\ oxygen = \frac{1.6}{32} = 0.05$
$Number\ of\ moles\ of\ nitrogen = \frac{1.4}{28} = 0.05$
$Total\ moles\ in\ the\ 10.0\ L\ cylinder\ at\ 27℃$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 0.1 + 0.05 + 0.05$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = 0.2\ mol$
$p_{T} = \frac{nRT}{V} = \frac{0.2 \times 0.082 \times 300}{10} = 0.492\ atm$