Advice on Complex Health Alterations

I'm really upset due to the fact I took my first test in Complex Health Alterations today and I studied for 10 hours over several days and tried to apply what I learned to different situations and I only got a 70 on the test. We didn't have very many questions, so it really lowered my grade. Our test was on electrolytes and acid-base. Any advice on how to study? I use a NCLEX book, the CDs and go to a study group. The class average was a 71 on the test so I know it wasn't just me. Other students have told me that this is the hardest test in this class is that so? Any advice will be appreciated.