A: Technically, air mixing should be done on a mass basis (ṁ₁H₁ + ṁ₂H₂ = ṁ₃H₃), but when the two streams are at similar temperatures, density doesn't vary much and volume flow rates are a close enough proxy for mass flow rates. The further apart the two streams are in temperature or humidity, the more you should consider doing a full mass-based mixing calculation. I'd encourage you to try it both ways on this problem while studying — you'll convince yourself the error is small and build your own judgment for when the shortcut is safe.