The Addition Rule of Probability
Compound Events
This video quickly explains what compound events are (as opposed to "simple events"), so that you can know what your professor and/or book are talking about when they mention compound events in the middle of confusing word problems.
The Addition Rule of Probability ("OR")
This chapter explains how to use the addition rule to calculate the probability of an "or" compound event. Sometimes it's obvious, such as "Calculate the probability of rolling a 3 OR a 6", other times there's an "or" in disguise, such as "Calculate the probability of rolling an even number", which is really "2 or 4 or 6."
Addition Rule for Complementary Events
This video covers a specific type of problem that's super-hard if you try to use the addition rule directly, but not nearly as bad if you use the complementary "1-P" trick. It's kind of like a "nor" problem, but I can't explain, just watch, you'll probably recognize it.