If a pipe can fill a tank in 4 hours and another pipe can empty the full tank in 6 hours, find the time required to completely fill the tank when both 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?