You have no doubt been noticing that the WebAssignments in Part 5 have been spaced out more widely than for the previous sections. That's because the assignments generally contain more questions, and the individual questions often require more thought. Most students view the "probability" part of the course to be the most demanding. There are few WebAssignments, but they are longer and require more thought. So that makes it more important than ever to get an early start on the assignments in Part 5. And do go back and review topics in Part 4 as needed. Often when a student claims to "not understand" probability, it turns out the that real difficulty lies with understanding the prerequisite concepts in Part 4.
Test #3 is the last test of the semester. Part 6 of the class is not covered on a test, but it is heavily represented on the final exam.