In this series, I’m compiling some of the quips and one-liners that I’ll use with my students to hopefully make my lessons more memorable for them.

At the end of every semester, instructors are often asked *“What do I need on the final to make a ___ in the course?”*, where the desired course grade is given. (I’ve never done a survey, but *A* appears to be the most desired course grade, followed by *C*, *D*, and *B*.) Here’s the do-it-yourself algorithm that I tell my students, in which the final counts for 20% of the course average.

Let be the grade on the **f**inal exam (as I write a big F on the chalkboard). [groans] After all, final starts with , and it’s important to assign variable names that make sense.

Also, let be the up-to-**d**ate course average prior to the final. [more groans]

This gives us the course average. Just to be nice, let’s call that . [sighs of relief]

So .

More seriously, here’s a practical tip for students to determine what they need on the final to get a certain grade (hat tip to my friend Jeff Cagle for this idea). It’s based on the following principle:

If the average of is , then the average of is . In other words, if you add a constant to a list of values, then the average changes by that constant.

As an application of this idea, let’s try to guess the average of . A reasonable guess would be something like . So subtract from all four values, obtaining . The average of these four differences is . Therefore, the average of the original four numbers is .

So here’s a typical student question: “If my average right now is an , and the final is worth of my grade, then what do I need to get on the final to get a ?” Answer: The change in the average needs to be , so the student needs to get a grade points higher than his/her current average. So the grade on the final needs to be .

Seen another way, we’re solving the algebra problem

Let me solve this in an unorthodox way:

This last line matches the solution found in the previous paragraph, .

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