M can do a piece of work in 12 days and N can do it in 24 days. They begin working together, but M leaves the work 6 days before its completion. Find the total number of days taken to 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?