We can write this as:
\[P_{total} = P_{air} + P_{acetone}\]
Given that the total pressure is $780 \, mmHg$ and the vapour pressure of acetone is $400 \, mmHg$, we can find the pressure of the air inside the drum as:
\[P_{air} = P_{total} - P_{acetone} = 780 \,
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