Quantitative Reasoning

I think I've got the basics of combinatorics down.

I just don't know when to add possibilites and when to multiply.

The fundamental counting principal says to multiply - but can someone outline an instance of where you would add up two permutations and a combination as opposed to just multiplying all 3 of the quotients?

I know to multiply when the events are separate and independent of each other but the MGMAT study guides don't have an instance where you would add but I'm sure I've run into an example of adding somewhere but I just forgot to write it down.

Thanks