P, Q and R, each working alone, can do a piece of work in 10 days, 15 days and 25 days respectively. If P works on all days, Q works on every odd-numbered day and R works on every even-numbered day, then in how many days will the whole 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?