A swimming pool has two inlet pipes, X and Y, and one outlet pipe, Z. Pipe X can fill the pool in 6 hours, and Pipe Y can fill it in 4 hours. Pipe Z can empty the pool in 12 hours. If all three pipes are opened at the same time, how long will it take to fill the pool?

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?