P can do a piece of work in 40 days and Q can do the same in 30 days. P started alone but left the work after 10 days. Q then worked at it for 10 days and left. R finished the remaining work in 10 days. In how many days can R alone 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?