A tyre has two punctures. The first puncture alone would have made the tyre flat in 45 minutes, and the second puncture alone would have done it in 90 minutes. If air leaks out at a constant rate, how long (in minutes) does it take for both punctures together to make the tyre flat?

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?