A and B can do a piece of work in 45 and 40 days respectively. They began the work together but A left the work after some days and B alone finished the remaining work in 23 days. After how many days did A leave?

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?