Work problems involves two or more persons or machines doing a particular task. In this type of problems, the rates of the persons or machines are usually given and the amount of time needed to complete a task is usually asked. Below are the posts that discuss in details these types of problems.
How to Solve Work Problems Part 1 discusses the details of the basic concepts like how the equations are formed and why the equations are equated to 1.
How to Solve Work Problems Part 2 discusses basic examples of work problems. It discusses how two hoses can fill a pool when they are opened simultaneously. It also discusses how a person can do a task alone given the rate of two persons working together.
How to Solve Work Problems Part 3 discusses two persons who worked together and after a while, the other person stopped. It also discusses the problem about how many hours a pool is filled if both the inlet and outlet pipes are open.