And this is its solution.
From this, it says that the density of air is 29/369 × 492/620. But it omitted the untis, so I can't understand the equation. I assume that this equation is probably ρ = PM/RT, since molecular weight of air is 28.97 g/mol when I searched for it. But the numbers 369, 492, 620 are still mystery. What is those?