The answer is 22.5.

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The first source fills $\frac{1}{30}$ of the pool per minute, and the second source fills $\frac{1}{90}$ of the pool per minute. Together, they can fill $\frac{1}{30} + \frac{1}{90} = \frac{4}{90}$ of the pool in each minute. Thus, the total number of minutes needed to fill the pool is $\frac{1}{\frac{4}{90}} = \frac{90}{4} = 22.5.$