Question 379614
An inlet pipe can fill a tank in 8 minutes. A drain can empty the tank in 12 mintes. If the tank is empty and both the pipe and drain are open, how long will it take before the tank overflows? 
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Let x = time (minutes) it takes to fill tank
then
x(1/8 - 1/12) > 1
multiplying both sides by 96:
x(12 - 8) > 96
x(4) > 96
x > 24 minutes
That is, tank begins to overflow after 24 minutes