A tank has two inlets, I1 and I2. I1 alone can fill it in 8 hours, and I2 alone can fill it in 4 hours. If both inlets are opened together, how long will it take to fill the tank?

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?