1. If A and B together can complete a piece of work in 15 days and B alone in 20 days, in how many days can A alone complete the work?

2. 7 men can complete a piece of work in 12 days. How many additional men will be required to complete double the work in 8 days?

3. One pipe fills a water tank three times faster than another pipe. If the two pipes together can fill the empty tank in 36 minutes, then how much time will the slower pipe alone take to fill the tank?

4. A can do a work in 12 days. When he had worked for 3 days, B joined him. If they complete the work in 3 more days, in how many days can B alone finish the work?

5. A and B can complete a piece of work in 8 days, B and C can do it in 12 days, C and A can do it in 8 days. A, B and C together can complete it in