The terms in my Probability section always throw me off a bit.

I just want to know if I am doing some of these correctly, because there are different rules for each i.e. conditional probability, Probability of "At Least One", Multiplication Rules/Addition, Replacement/Without Replacement(those 2 I kind of get). Sometimes I am unsure of which steps to do because some of the terms I am not yet fully comfortable with.

For example, I have this question that States, What is the Probability that If a Single student is Randomly Selected they would have a GPA Greater than or equal to 3.0.. What I did, was look at at those who had a GPA that was higher than 3.0, and added them up, then divided them by the total number of students. You know P(A)/ Number of times A occured/Total Number. I am just wondering if I did this right because it seems I have to do a mix of everything.

I recall my profe, she told me, that there is a rule if it's a single trial you do one step, and if it's more then 1 you do another. I think that was the multi rule.

Sorry for the confusion, I am going through my book trying to take a ton of notes and figure out certain rules and steps. Thanks.