Not bad... but unfortunately the part it leaves out is critical (at least for me). How do you get that formula at the end? This, unfortunately, is the problem

Not bad... but unfortunately the part it leaves out is critical (at least for me). How do you get that formula at the end? This, unfortunately, is the problem with a lot of "make math easy" articles. The topic itself is not easy, so at some point, you need to bring in tough math. What I dislike is that the article doesn't make it clear how critical that bit is.