A is twice as good a workman as B, and B is twice as good a workman as C. If A and B can together finish a piece of work in 4 days, then C can do it by himself in:

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?