It would take one machine 5 hours to complete a large production order and another machine 4 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?
Choose the correct answer
A) A 9/20 B) B 20/9 C) C 7/19 D) D 5/21 E) E 3/4
The rate of the first machine is 1/5 job per hour;   The rate of the second machine is 1/4 job per hour;   Thus, the combined rate of the machines is 1/5+1/4=9/20 job per hour, which means that it takes 1/(9/20)=20/9 hours both machines to do the job.   Answer: B 20/9.
<result>B</result>
It would take one machine 5 hours to complete a large production order and another machine 4 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?