Two taps, R and S, can fill a cistern separately in 30 minutes and 40 minutes, respectively. They are opened together to fill the cistern, but tap R is turned off after a few minutes, and tap S fills the remaining part of the cistern in 5 minutes. After how many minutes was tap R turned off?

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?