A, B and C can complete a piece of work individually in 7.5 days, 15 days and 30 days, respectively. A and B start working together but A quits the job after 3 days. Then C joins B and both work together until the completion of the work. In how many days will the entire work be completed?

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?