It would take one machine 7 hours to complete a large production order and another machine 6 hours to complete the same order. How many hours would it take both machines, working simultaneously at their respective constant rates, to complete the order? Options:  A) A 13/42  B) B 12/42  C) C 11/42  D) D 10/42  E) E 42/13
The rate of the first machine is 1/7 job per hour;   The rate of the second machine is 1/6 job per hour;   Thus, the combined rate of the machines is 1/7+1/6=13/42 job per hour, which means that it takes 1/(13/42)=42/13 hours both machines to do the job.   Answer: D 10/42.
<result>D</result>
It would take one machine 7 hours to complete a large production order and another machine 6 hours to complete the same order. How many hours would it take both machines, working simultaneously at their respective constant rates, to complete the order?