A pipe can fill a cistern in 18 hours. Due to a leak in its bottom, the cistern fills up in 20 hours. If the cistern is full, in how much time will it be completely emptied by the leak?

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?