Question 1192357
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A water tank can be filled in 9 hours and then drained in 12 hours. 
How long would it take to fill the water tank if the drain is left open?
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<pre>
Filling  rate is  {{{1/9}}}  of the tank volume per hour.

Draining rate is  {{{1/12}}}  of the tank volume per hour.

The net filling rate is the difference  {{{1/9 - 1/12}}} = {{{4/36 - 3/36}}} = {{{1/36}}}.


Hence, the tank will be filled in 36 hours, if the drain is left open.
</pre>

Solved.