\textbf{171-} In an open space, the sound intensity level at a distance of $50$ meters from a sound source is $60$ decibels. How many milliwatts is the power of the source? $\left(I_0 = 10^{-12}\,\dfrac{\text{W}}{\text{m}^2},\ \pi = 3,\ \text{and energy absorbed by the medium is ignored.}\right)$
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(1) $0.3$ \qquad (2) $6$ \qquad (3) $7.5$ \qquad (4) $30$
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\textit{Calculation Space}
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