Machines A and B produce bottles at their respective constant rates. Machine A produces k bottles in 4 hours and machine B produces k bottles in 5 hours. How many hours does it take machines A and B, working simultaneously, to produce 18k bottles?	Choose the correct choice from the following answers:
A) 28
B) 32
C) 36
D) 40
E) 44
A's rate = k/4   B's rate = k/5   k/4 + k/5 = 9k/20   18k / (9k/20) = 40 hours   The answer is 40.18  / (9 /20)= 
<gadget id="calculator">18  / (9 /20)</gadget>
<output>40</output>
40 hours   The answer is 40.
<result>D</result>
Machines A and B produce bottles at their respective constant rates. Machine A produces k bottles in 4 hours and machine B produces k bottles in 5 hours. How many hours does it take machines A and B, working simultaneously, to produce 18k bottles?