A can do a piece of work in 10 days and B can do the same piece of work in 20 days. They work on alternate days, starting with A on the first day. How many days will it take them to complete 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?