To do the exercise, fork this, clone your fork to your computer, make changes to mixed.R, knit it (to create output), git commit -m"my homework is done" -a them, git push, and then in github make a pull request. See https://guides.github.com/activities/forking/ for how to do this.

Note that this uses the yearn package to make this work more easily for teaching (rather than have students hunt down which packages they still need to get, it just loads those they don't have).