Hema can do a certain piece of work in 30 days. Hema and Raji can together do the same work in 25 days, and Hema, Raji and Smitha can do the same work together in 10 days. In how many days can Hema and Smitha do the same work together?

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?