rokufawx1
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One pipe can fill a tank in 36 minutes, a second can fill it in 9 minutes, and a third can fill it in 12 minutes. If the tank is empty, how long will the three pipes, operating together, take to fill it?

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rlspike1
 i think that the answer is 1/x
irspow
The filling rates of each pipe are 1/36, 1/9 and 1/12 respectively.  So if all pipes operated together to fill the tank:

t/36+t/9+t/12=1  making all terms have a common denominator of 36...

(t+4t+3t)/36=36/36

8t=36

t=4.5 minutes  (4 min 30 sec)

So it would take all three pipes four and a half minutes to fill the tank.

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