Working alone, printers X, Y, and Z can do a certain printing job, consisting of a large number of pages, in 15, 12, and 18 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?
Options.
A) 4/15  B) 1/2  C) 15/22  D) 25/12  E) 11/4
The time it takes printer X is 15 hours.   The combined rate of Y and Z is 1/12 + 1/18 = 5/36   The time it takes Y and Z is 36/5   The ratio of times is 15 / (36/5) = 5*15/36 = 25/12   The answer is 25/12.
<result>D</result>
Working alone, printers X, Y, and Z can do a certain printing job, consisting of a large number of pages, in 15, 12, and 18 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?