Once we have mastery of the basics of mental math, we will branch out in interesting directions. Lecture 5 offers techniques for easily finding approximate answers when we don’t need complete accuracy. Lecture 6 is devoted to pencil-and-paper mathematics but done in ways that are seldom taught in school; we’ll see that we can simply write down the answer to a multiplication, division, or square root problem without any intermediate results. This lecture also shows some interesting ways to verify an answer’s correctness. In Lecture 7, we go beyond the basics to explore advanced multiplication techniques that allow many large multiplication problems to be dramatically simplified.