Pipe A can fill a cistern in 36 minutes and pipe B can fill it in 48 minutes. If both the pipes are opened together and the cistern is full in 24 minutes, then after how many minutes was pipe B closed?

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?