Three pipes X, Y, and Z can fill a tank in 12, 15, and 20 hours respectively. All three pipes are opened together, but pipe Z is closed 3 hours before the tank is completely filled. In how many hours will the tank be filled?

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?