Alarge pump can drain an 80,000 gallon pool in 3 hours. with a smaller pump also operating, the job only takes 2 hours. how long would it take the smaller pump to drain the pool by itself?

We know that

A large pump can drain an 80,000 gallon pool>  in 3 hours
 X gallons> in 2 hour
X=2*80000/3> 53333.33 gallons in 2 hour

therefore
the smaller pump in two hours drain an [80000-53333.33]=26666.67 gallons

if the smaller pump drain 26666.67 gallons> in 2 hours
 80000 gallons> X
X=80000*2/26666.67> 6 hours

the answer is 
the smaller pump to drain the pool by itself would take 6 hours