There are probably a lot of things you learned in school that you don't even remember, but the "order of operations" -- also known as PEMDAS -- is likely to be one that stuck with you; you'll mess up even simple equations without it. The catch? Well, it's wrong.
The problem, as MinutePhysics explains, is that the real process of solving equations that have multiple operators in them has more to it than your average kid is ever led to believe. PEMDAS works most of the time, but it only approximates the real logic behind the scenes. Now that you're all-grown up, maybe it's time to learn the truth. Don't worry, it's not that horrifying. [MinutePhysics]