Pipe A can fill a tank in 13 hours, Pipe B in 27 hours, and Pipe C in 19 hours. What is the total time taken to fill the tank when all three pipes are opened 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?