A can complete a piece of work in 25 days and B can complete it in 23 days. If they work on alternate days, starting with B on the first day, how long will they take (in days) to finish the work?

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?