There are three taps which are either filling a tank or emptying it. If all three taps are opened together, the tank is filled in 10 hours. One of the taps can fill the tank in 5 hours and another can fill it in 10 hours. What does the third tap do?

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?