Working alone at its constant rate, machine K took 3 hours to produce 1/8 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?  Select
A) 9.6
B) 12.4
C) 16.2
D) 18.3
E) 21.5
Machine K works at a rate of 1/24 of the units per hour.   The rate of K + M together is 7/8 * 1/6 = 7/48 of the units per hour.   The rate of machine M is 7/48 - 2/48 = 5/48.   It would have taken machine M a total time of 9.6 hours.   The answer is 9.6.
<result>A</result>
Working alone at its constant rate, machine K took 3 hours to produce 1/8 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?