'A' can do a piece of work in 30 days and 'B' alone can do it in 20 days. They work together for 4 days and then 'B' leaves the job. 'A' will finish the remaining work in:

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?