In this video, learn how to find the greatest common factor of two numbers. Then, see how this can be used to solve problems with large numbers. In the accompanying classroom activity, students learn to find prime factors using factor trees. They then learn how prime factors, a Venn diagram, and simple multiplication can help them find the greatest common factor of two numbers. The lesson challenges students to find the greatest common factor of increasingly large numbers. It also pushes them to think about how prime factors are the building blocks of composite numbers.