It would take one machine 6 hours to complete a large production order and another machine 5 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? Select:
A) A 11/30  B) B 11/20  C) C 30/11  D) D 9/10  E) E 4/10
The rate of the first machine is 1/6 job per hour;   The rate of the second machine is 1/5 job per hour;   Thus, the combined rate of the machines is 1/6+1/5=11/30 job per hour, which means that it takes 1/(11/30)=30/11 hours both machines to do the job.   Answer: C 30/11.
<result>C</result>
It would take one machine 6 hours to complete a large production order and another machine 5 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?