Raju, Sunil, and Vishal can separately finish a work in 20, 30, and 40 days, respectively. In how many days can Raju finish the work if he is assisted by Sunil and Vishal on alternate days, starting with Sunil on the first day?

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?