Question 1134494
Let's see what happens under a minute. 

Filling rate = 1/12min
Draining rate = - 1/30min

Together: 1/12-1/30 = 1/x, where x represents the time it takes to fill the tub.

{{{1/12-1/30=1/x}}}
{{{5/60-2/60=1/x}}}
{{{3/60=1/x}}}
{{{1/20=1/x}}}
{{{x=20}}}

It would take 20 minutes to fill the tub.

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