A and B can complete a piece of work in 8 days and 12 days, respectively. A starts working on the task alone and after 3 days, B joins him. How many more days will it take to finish the work after B joins?

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?