Would you rather go to a college with a high teacher-to-student ratio or a low one? What about the ratio of girls-to-boys? What is the ratio of eggs to butter in your favorite dessert? Ratios show up EVERYWHERE in life. This tutorial introduces you to what they (and proportions) are and how to make good use of them!

How fast can a robot possum fly? What is the rate at which a hungry you can consume avocados? This tutorial helps you make sense of these fundamental questions in life.

Wait, I'm in Europe and my car only tells my distance traveled in kilometers! But I'm used to a units of distance devised by the Romans to measure the average length of 1000 paces of a soldier (the "mile")! How do I operate? This tutorial is about the fundamental skill of unit conversion. Sal's cousin Nadia being a bit confused with this was actually the reason why he started tutoring her (which led to the creation of the Khan Academy).