Solving 3 Gas Mixture: Assumptions & PV=mRT

[edwin.07]

1. A rigid container is filled with a mixture of three gases: A, B and C. the pressure gauge reads P[T]. The container is evacuated and fill with an amount of gas A equal to that in the original mixture. The gauge now reads P[A]. This process is repeated for gases B and C with the pressure gauge reading P and P[C]? What assumptions are made?



2. PV=mRT



3. The gases are related by the volume because they might have different pressures, temperatures and mass.

 

[mfb]

I don't see any assumptions made (apart from "the container is filled with gas 4 times"), as there is no analysis described.
I guess an analysis will assume that volume is constant, and it might be a reasonable assumption that all gases are ideal.

What is the unit for "amount of gas" to compare different gases?