Working alone, printers X, Y, and Z can do a certain printing job, consisting of a large number of pages, in 12, 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 the correct choice from the following:
A) 5 / 3
B) 7 / 4
C) 9 / 5
D) 11 / 6
E) 13 / 7

Rationale: "the time it takes printer x is 12 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 12 / ( 20 / 3 ) = 3 * 12 / 20 = 9 / 5 the answer is c ."
Answer: C