This one is a straightforward mistake. They just switched the sign of their equation.
This question is pretty dumb. I don't see any reason why you couldn't relate any of these variables together in an equation. Particularly, I think it would be fun to relate Athena's level of thirstiness and how much cash she's willing to put into the machine.
This exercise says that there are three incorrect answers, but there are actually four wrong answers.
Here's a question that's kind of dumb, but a kid pops up in the forum and makes it a thousand times more interesting:
Anyway, based on what you read in the news, you might've come to the conclusion that Udacity's College Algebra course isn't very good. Turns out? Yeah, it's sloppy and not very good.
Shocking, I know.