A takes as much time to do a piece of work as B and C together take to complete it. If A and B together can finish the work in 10 days, and C alone can finish it in 20 days, in how many days will B alone 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?