Three pipes are attached to a tank. The first one fills it completely in 5 hours, the second one fills it completely in 4 hours, and the third one empties it in 20 hours. If all three pipes are opened simultaneously at 7 a.m., at what time will the tank be completely 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?