Pipe A can fill an empty tank in 18 hours and pipe B can fill it in 24 hours. If both the pipes are opened together and after 3 hours pipe A is closed, how much time (in hours) will pipe B take to fill the remaining part of the tank?

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?