Dagny has been telling me smugly that it’s now my turn to write an article. So I decided to write one as soon as possible and pass the buck back to her. We both know that it takes a lot of pain to research for an article and write the text so we both try to avoid it for as long as possible. On the other hand, since we both enjoy writing and like to publish a well-finished chapter, we both undertake the task eagerly once the ball is in our court. Writing these chapters have made me realize how true is the maxim “if you want to learn something well, teach it.” I can cite a lot of topics which I learnt while teaching in the class or answering students’ questions. These years of teaching have made so many topics seem pretty childish to me which were nightmares to me before. But knowing a topic well and teaching it are two different ball games. Many a times, I have found myself struggling in the class to make a student understand something which I have found obvious to understand! The fault is not on the student’s part. Three or four years ago, I would have been stuck on the same point! Every time I encounter a situation like this, my only measure is to stop at that point, retrace my steps with the students and slowly unravel the difficulty he’s facing. It works most of the times. But sometimes, the student is in so much awe with the topic that his mind gets frozen. I have seen this happening many a times. Topics such as time, speed and distance, Permutation and Combination, geometry etc. inspire so much fear among the students that sometimes simple principles, which they would have otherwise understood do not strike them as simple. I get incessant queries such as “Sir, time speed distance chhod sakte hain?,” “Sir, permutation combination na karein to chalega?” from my students all the time. And the sad part is, that the level of CAT in these topics is very simple. They are doable. In the present chapter, I am trying to present one of these dreaded chapters as I understand it. I hope my students understand it too.