Question 608581
    a tank can be filled by three pipes separaely in 20, 30, and 60 minutes respectively. In how many minutes can it be filled by the three pipes working together?
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Let x = time (minutes) it takes for all three
then
x(1/20 + 1/30 + 1/60) = 1
multiplying both sides by 60:
x(3 + 2 + 1) = 60
x(6) = 60
x = 60/6
x = 10 minutes