If a pump can fill a pool within 6 hours, and another pump can empty the pool within 3 hours, how long will it take the pool to empty when it's full and when both pumps are working?

6 hours

Step-by-step explanation:

Pump 1 velocity = 1 pool / 6 hours = 1/6 [pool/hour]

Pump 2 velocity = 1 pool / 3 hours = 1/3 [pool/hour]

Assuming two pumps are running the resultant velocity = 1/3 - 1/6 = 1/6 [pool/hour]

So it will take 6 hours