Machine A working alone can complete a job in 12 hours. Machine A+B working alone can do the same job in 15 hours. How long will it take machine B working at their respective constant rates to complete the job?  Choose the correct answer
A) 80 B) 70 C) 60 D) 50 E) 90
Machines ( A ) ( B ) (A+B)   ---Time - 12 - ( - ) -(15)   ---Rate 5 --- 1 -- 4   --Work 60 --60 - 60   A+B = (60/1)= 
<gadget id="calculator">(60/1)</gadget>
<output>60</output>
60   60
<result>C</result>
Machine A working alone can complete a job in 12 hours. Machine A+B working alone can do the same job in 15 hours. How long will it take machine B working at their respective constant rates to complete the job?