Most students find the 3rd test of the semester (on probability) to be the most challenging. So don't get caught by surprise. Give yourself adequate time to study for this test. And review Part 4 as needed since the techniques from Part 4 are heavily used in Part 5. What makes Part 5 tougher is that probability techniques fit so many different types of scenarios. So you may have to spend more time figuring out a proper approach for questions than you've been accustomed to in the previous sections.