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The total pressure in a gas is the sum of the partial pressures of the components, that, at the same temperature and constant volume, are proportional to the molar fractions of the components themselves.

The mixture of $"CO"_2$ and $"N"_2$ gases, after absorption of $"CO"_2$ by $"CaO"$, drops by $"520 torr"$, which corresponds to the proportion of $"CO"_2$ over the total gas.

$520/820=x/0.2 $

So

$x = "0.124 moles"$

of $"CO"_2$ were present in the original mixture.