Pipe A can fill a tank in 17 hours, pipe B can fill the same tank in 21 hours, and pipe C can fill it in 19 hours. Find the time taken by them to fill the tank if all three pipes operate 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?