If 15 men take 6 days to complete a job working 5 hours a day, how many hours a day should 18 men work to complete the same job in 6 days?

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?