Machines A and B produce bottles at their respective constant rates. Machine A produces k bottles in 2 hours and machine B produces k bottles in 4 hours. How many hours does it take machines A and B, working simultaneously, to produce 15k bottles? Choose the correct choice
A) 10  B) 15  C) 12  D) 20  E) 18
A's rate = k/2   B's rate = k/4   k/2 + k/4 = 3k/4   15k / (3k/4) = 20 hours   The answer is 20.15  / (3 /4)= 
<gadget id="calculator">15  / (3 /4)</gadget>
<output>20</output>
20 hours   The answer is 20.
<result>D</result>
Machines A and B produce bottles at their respective constant rates. Machine A produces k bottles in 2 hours and machine B produces k bottles in 4 hours. How many hours does it take machines A and B, working simultaneously, to produce 15k bottles?