Two pipes A and B can fill a tank in 18 hours and 26 hours respectively. If they are opened at alternate hours and if pipe A gets opened first, then in how many hours will the tank get full?

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?