A can do a certain work in 20 days and B can do the same work in 24 days. A works alone for 1 day, then B works alone for 2 days, then A works alone for 3 days, then B works alone for 4 days, and so on alternately. On which day will the work be completed?

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?