A, B and C can complete a piece of work in 10, 15 and 30 days respectively. They all start working together, but after 3 days A leaves the work, and after another 3 days B leaves the work. In how many days will C complete the remaining part of the work?

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?