Economists like to weigh the benefits and costs of an activity to find the optimal amount of time that should be spent on it. While entering the Community Y this morning, I held the door open for a man entering behind me. I wondered if I would have held the door open if the man would have been farther behind me. I calculated my utility function as a constant 2 utils, and my cost function as .5X^3. I immediately plotted the the two and derived the graph you see. As you can see, I won't wait long.
I wrote this post as a way for my students to see marginal decision making. I want them to make something like it to make their learning relevant.
Some other ideas are how long should you study, workout, watch television, and write a blog post.