In this lecture, students learn how to decide whether to use HCF (Highest Common Factor) or LCM (Least Common Multiple) while solving word problems. The HCF is used when we need to divide or share things equally without any leftover, like making equal groups or packets. The LCM is used when we are looking for things to happen together again or at the same time, like finding the next time buses arrive together. Through examples and exercises, students understand the key clues in questions that tell whether to apply HCF or LCM, and they practice solving real-life problems using these methods.