I didn't want to derail another thread or insult the person who started it, so I'm starting this one. In the other thread the figures used in the question make no real world sense. No one would balance an HVAC system to a 100th of a gallon. Real world, you'd be lucky to get 1/2 a gallon. Nor would they be likely to use the temperatures stated in the problem as a design state. I'm guessing this was an engineering school question, but I could be wrong. Have you ever come across any questions in a school book or on a test that just make no real world sense?