These two types of problems always involve two letters (usually x and y) and usually start the same way: if the problem has the words "varies directly" in it (or says two things are "proportional"), you just write down "y=kx" and start plug-and-chugging. If the problem says "varies inversely" (or variables are "inversely proportional"), you write down y=k/x and start plug-and-chugging. Sound too easy? What can I say: the same four steps will get you through every one of these things.
If you do not have an account, you should get one, because it is awesome! You can save a playlist for each test or each chapter, and save your "greatest hits" into a "watch right before the final" list (not that we recommend cramming, but when in Rome...)