Gas is being pumped into a spherical balloon at the rate of $ 30 \, \text{ft}^3/\text{min} $. Then the rate at which the radius increases when it reaches the value $ 15 \, \text{ft} $ is:
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In related rates, always identify your "knowns" and your "unknowns" before differentiating. Remember that $ \frac{dV}{dt} = (\text{Surface Area}) \times \frac{dr}{dt} $ for a sphere.