In Part 5 the sections on uniform probability distributions (the 5.3 WebAssignments) and the sections on conditional probability (the 5.4 and 5.5 WebAssignments) are the peak of the course. These assignments are due tomorrow and next week. Then Part 5 concludes with two special topics that aren't so challenging (independence and independent trials in 5.6 and expected value in 5.7). In other words, by the time you finish 5.5, you've passed the toughest part of the course and it's down hill from there on.
Part 6 will require some work, but it's not as conceptually demanding as Part 5. That's because the situations fit only a few different patterns, and therefore you don't have to spend a lot of time deciding how to approach a question.