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This problem can be solved in different ways.
Let x be the time the faster pump can drain the pool working alone.
Then the time for the slower pump is 4x, according to the condition.
In one hour (in each hour) the faster pump will drain of the pool volume, while the slower pump will drain of the pool volume.
Working together, these pumps will drain of the pool volume per hour.
= = .
From the condition,, we know that this sum is equal to of the tank volume.
Hence, = , which implies x = hours = hours = hours = 3 hours and 45 minutes. ANSWER
Solved.
Another way is THIS :
The faster pump works as effectively / (productively) as 4 slower pumps.
Therefore, the problem is equivalent, as if 4+1 = 5 slower pumps work simultaneously.
So, then we know that 5 slower pumps drain the pool in 3 hours.
Hence, one slower pump drains the pool in 3*5 = 15 hours.
The faster pump makes it in 4 times faster, i.e. in hours = 3 hours and 45 minutes.
You get the same answer.