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 7 hours. How many hours does it take machines A and B, working simultaneously, to produce 22k bottles?  Answers.
A) 48
B) 52
C) 56
D) 60
E) 64
A's rate = k/4   B's rate = k/7   k/4 + k/7 = 11k/28   22k / (11k/28) = 56 hours   The answer is 56.22  / (11 /28)= 
<gadget id="calculator">22  / (11 /28)</gadget>
<output>56</output>
56 hours   The answer is 56.
<result>C</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 7 hours. How many hours does it take machines A and B, working simultaneously, to produce 22k bottles?