Machine A working alone can complete a job in 3 hours. Machine B working alone can do the same job in 4 hours. How long will it take both machines working together at their respective constant rates to complete the job?  Choose the correct answer
A) 1.10 min	B) 1.714 hr	C) 4hr	D) 2hr	E) 1.714 hr
Machines ( A ) ( B ) (A+B)   ---Time - 3 - ( 4 ) 1.714   ---Rate 4 --- 3 ---7   --Work 12 ---12 ---12   1.714 hr
<result>E</result>
Machine A working alone can complete a job in 3 hours. Machine B working alone can do the same job in 4 hours. How long will it take both machines working together at their respective constant rates to complete the job?