Machines X and Y produce bottles at their respective constant rates. Machine X produces k bottles in 6 hours and machine Y produces k bottles in 3 hours. How many hours does it take machines X and Y , working simultaneously , to produce 12k bottles?	Choose the correct choice from the following answers.
A) 4
B) 8
C) 12
D) 18
E) 24
x rate = k/6   y rate = k/3   k/6 + k/3 = 12k / T   solving T = 24   Answer 24
<result>E</result>
Machines X and Y produce bottles at their respective constant rates. Machine X produces k bottles in 6 hours and machine Y produces k bottles in 3 hours. How many hours does it take machines X and Y , working simultaneously , to produce 12k bottles?