To complete a piece of work, A takes half the time taken by B. C completes the same work in the same time in which both A and B complete it together. If all three together complete this work in 7 days, then in how many days will C complete this work alone?

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?