Machine A working alone can complete a job in 9 hours. Machine B working alone can do the same job in 12 hours. How long will it take both machines working together at their respective constant rates to complete the job?
Choose the correct answer.
A) 21/8
B) 27/5
C) 12/7
D) 36/7
E) 15/7
Machines ( A ) ( B ) (A+B)   ---Time - 9 - ( 12 ) -   ---Rate 4 --- 3 --- 7   --Work 36 ---36 --36   A+B = (36/7)   36/7
<result>D</result>
Machine A working alone can complete a job in 9 hours. Machine B working alone can do the same job in 12 hours. How long will it take both machines working together at their respective constant rates to complete the job?