If I were you, I would have all the combustion problems figured out before the exam. Like all the molecular weights for each component that you see, co, co2, h20, ect..... I would pre-figure out all the fuels methane, propane, and know how to calculate the moles for exhaust gases stoiciometrically ....... Know how to calculate the percent water vapor in the flue gas or any other component.
Practice these, I personally struggled through these. These are the problems I panic on. Was never comfortable with them. I skipped all my exam PROBLEMS of this type and was lucky to pass 1st time.