A and B can do a certain work in 27 days and 36 days, respectively. A worked for a few days alone and then B joined him. They together worked till the completion of the work. If the work got completed in a total of 18 days, then for how many days did A and B work together?

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?