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?  Select the correct option
A) 4 / 15 B) 1 / 2 C) 15 / 22 D) 25 / 12 E) 11 / 4

Rationale: "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 d ."
Answer: D