Pipe A can fill a tank in 16 hours, pipe B can fill the same tank in 21 hours and pipe C can fill the same tank in 15 hours. The time taken by them to fill the same tank if they operate together is:

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?