A man, a woman and a boy can together complete a piece of work in 3 days. If a man alone can do it in 6 days and a boy alone in 18 days, how long will a woman alone take to complete the work?
Answer: A
Combined one day's work = 1/3. Man's one day work = 1/6. Boy's one day work = 1/18. Woman's one day work = \(1/3 - (1/6+1/18) = 1/3 - (3+1)/18 = 1/3 - 4/18 = 1/3 - 2/9 = (3-2)/9 = 1/9\). A woman alone will take 9 days.