one pump can drain a pool in 12 minutes. When the second pump is also uses, the pool only takes 3 minutes to drain. How long would it take the second pump to drain the pool if it were the only pump in use

Steve & Jim are correct, but perhaps this can help you think about the relationship between the pumps & water:

If the first pump takes 12 minutes alone, and both pumps together do it in 3 minutes, then we know that the first pump empties 1/4 of the pool in the time it takes the second pump to empty 3/4 of the pool. Emptying 3/4 of a pool in 3 minutes is the same as 1/4 of the pool each minute. Therefore, the second pump does the job in 4 minutes.

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