Answer:
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
