MIT's 18.200 Principles of Discrete Applied Mathematics, taught by Ankur Moitra, devotes this session to the inclusion-exclusion principle. Moitra states and proves the principle, then applies it to the classic hat check problem, where the question is how many ways a coat check clerk can return hats so that nobody gets their own back. From there the lecture moves to the coupon collector's problem, working out the expected number of draws needed to collect every type of coupon in a set. The session closes by defining independence of random variables, laying groundwork for the probability tools used later in the course. The style is chalkboard and formula driven, with Moitra building each proof step by step rather than skipping to results. At seventy nine minutes, it covers enough ground to function as a self contained unit on counting and basic probability for anyone studying discrete math.