Working alone, printers X, Y, and Z can do a certain printing job, consisting of a large number of pages, in 16, 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?  Choose one
A) 7/4  B) 10/3  C) 12/5  D) 15/7  E) 18/11
The time it takes printer X is 16 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 16 / (20/3) = 3*16/20 = 12/5   The answer is 12/5.
<result>C</result>
Working alone, printers X, Y, and Z can do a certain printing job, consisting of a large number of pages, in 16, 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?