Input and output rate question

Question

Input: It would would take 12 hrs to fill the pool.

Output: It would take 3g/4h to empty the pool in 48 hrs.

I'm not sure how to approach this question.

Answer 1

If the pool must be filled to $36$ gallons in $48$ hours, the water must be flowing in at a net rate of $3/4$ gallon per hour. The input supplies $3$ gallons, so the output must be $3 - 3/4 = 9/4$ gallons per hour.