Working alone, printers X, Y, and Z can do a certain printing job, consisting of a large number of pages, in 15, 10, and 20 hours, respectively. What is the ratio of the time it takes printer X to do the job, working alone at its rate, to the time it takes printers Y and Z to do the job, working together at their individual rates?
Pick:  A) 5/2  B) 8/3  C) 9/4  D) 10/3  E) 12/5
The time it takes printer X is 15 hours.   The combined rate of Y and Z is 1/10 + 1/20 = 3/20   The time it takes Y and Z is 20/3   The ratio of times is 15 / (20/3) = 3*15/20 = 9/4   The answer is 9/4.
<result>C</result>
Working alone, printers X, Y, and Z can do a certain printing job, consisting of a large number of pages, in 15, 10, and 20 hours, respectively. What is the ratio of the time it takes printer X to do the job, working alone at its rate, to the time it takes printers Y and Z to do the job, working together at their individual rates?