Question: a can do a piece of work in 7 days?
Answer: A can do a piece of work in 7 days of 9 hr each or in 63 hrs. B can do it in 6 days of 7 hr each or 42 hrs. So, in 1 hour, A can do 1/63 of the work and B can do 1/42 of the work. Together, they can finish 5/126 of the work in 1 hour or they work gets completed in 126/5 hrs.