Working alone at its constant rate, machine K took 3 hours to produce 1/5 of the units produced last Friday. Then machine M started working and the two machines, working simultaneously at their respective constant rates, took 6 hours to produce the rest of the units produced last Friday. How many hours would it have taken machine M, working alone at its constant rate, to produce all of the units produced last Friday?
Choose the correct choice from the following:
A) 10 B) 12 C) 15 D) 25 E) 30
Machine K works at a rate of 1/15 of the units per hour.   The rate of K + M together is 4/5 * 1/6 = 2/15 of the units per hour.   The rate of machine M is 2/15 - 1/15 = 1/15.   It would have taken machine M a total time of 15 hours.   The answer is 15.
<result>C</result>
Working alone at its constant rate, machine K took 3 hours to produce 1/5 of the units produced last Friday. Then machine M started working and the two machines, working simultaneously at their respective constant rates, took 6 hours to produce the rest of the units produced last Friday. How many hours would it have taken machine M, working alone at its constant rate, to produce all of the units produced last Friday?