If A can do a piece of work in 20 days and B can do it in 30 days, how much work will they complete together in 10 days?
Answer: A. 1/2 of the work
A's rate is 1/20 and B's rate is 1/30. Together, they complete 1/20 + 1/30 = 1/12 of the work per day. In 10 days, they complete 10/12 = 5/6 of the work.