Ram, Ravi and Reena can do a piece of work in 16, 20 and 24 days, respectively. They began the work together but Ravi left the work 5 days before the completion of the work. In how much time did they finish the work together?

If P and Q together can complete a piece of work in 15 days and P alone can do it in 45 days, in how many days can Q alone complete that work?

A can lay a railway track between two stations in 16 days and B can do the same job in 12 days. With the help of C, they finished the job in 4 days. In how many days can C alone do the job?