A can finish a work in 20 days, while B can finish it in 30 days. If they work together for 10 days, how much of the work is left?
Answer: A. 1/3
In 10 days, A completes 10/20 = 1/2 of the work and B completes 10/30 = 1/3 of the work. Together they complete 1/2 + 1/3 = 5/6 of the work, leaving 1 - 5/6 = 1/6 of the work left.