The author in this video explains the concept of fundamental rule of proportions. Fundamental rule of proportions means cross multiply. He explains that to arrive from an equation which has fractions into the one without equation multiply the top of left side with the bottom of right side and equal it with bottom of left side multiplied with top of the right side. In this way proportions can be cross multiplied to arrive at an algebraic expression. If you want to master the rule of proportions watch this easy to follow video.
Have an iPhone? Check out all 200+ new features coming in iOS 13.