Ravi and Raju can do a piece of work separately in 12 days and 20 days, respectively. If they work on alternate days, with Ravi starting on the first day, then in how many days will they complete the entire 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?