If Steven can mix 20 drinks in 5 minutes, Sue can mix 20 drinks in 10 minutes, and Jack can mix 20 drinks in 15 minutes, how much time will it take all 3 of them working together to mix the 20 drinks?
(time) x (total mixing rate) = 20 drinks
t x [(20/5) + 20/10 + 20/15]= 20
t x [1/5 + 1/10 + 1/15) = 1
t x {6/30 + 3/30 + 2/30) = 1
t = 1/(11/30) = 30/11 minutes
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