Question 161735
 person A can fill a tank in 10 minutes
person B can fill a tank in 20 minutes
person C can fill a tank in 30 minutes 
how long will it take to fill the tank if they all work together?
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Add the reciprocals, then invert it.
1/10 + 1/20 + 1/30 = 11/60
The time is 60/11 minutes, or 5 5/11 minutes.