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 12 hours. How many hours does it take machines X and Y , working simultaneously , to produce 12k bottles?	Choices
A) 8
B) 12
C) 15
D) 48
E) 24
x rate = k/6   y rate = k/12   k/6 + k/12 = 12k / T   solving T = 48   Answer 48
<result>D</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 12 hours. How many hours does it take machines X and Y , working simultaneously , to produce 12k bottles?