... Ms. Duff wants to give her students some apples. If she gives each student 7 apples, there will be no apples left. If she gives each student 4 apples, there will be 18 apples left. How many students are in Ms. Duff’s class? ...

Alfred Marshall (26 July 1842 – 13 July 1924) was one of the most influential economists of his time. His book, Principles of Economics (1890), was the dominant economic textbook in England for many years. It brings the ideas of supply and demand, marginal utility, and costs of production into a coherent whole. He is known as one of the founders of economics.