Time and Work - General Questions

A can do a piece of work in 9 days and B can do it in 21 days. In how many days will they finish it working together?

A and B together can complete a work in 4 days. A alone can complete it in 12 days. In how many days can B alone complete the work?

A can do a piece of work in 6 days and B can do it in 16 days. In how many days will they finish it working together?

A can do a piece of work in 16 days and B can do it in 12 days. In how many days will they finish it working together?

A is twice as efficient as B. Together they finish a job in 10 days. How long would A take to do it alone?