The WebAssignments from 5.3 to 5.5 are really the heart of the course. These sections make heavy use of the counting techniques from Part 4. And they introduce the concept of conditional probability. And Section 5.5 connects conditional probability with tree diagrams to give a good way of visualizing certain conditional probability considerations. Sections 5.6 and 5.7 deal with two special topics, i.e. independent trials and expected value. But generally I'd say that the most challenging part of the course is the middle of Part 5, and from there on things ease up a bit as we approach the end of the course.