Answer
$\mathcal{V}_{avg}=\frac{2}{R^2}\int_0^R \mathcal{V}(r) rdr$
Work Step by Step
$\dot{m}=\rho\pi R^2\mathcal{V}_{avg}=\int_0^R \rho \mathcal{V}(r)2\pi rdr$
$\mathcal{V}_{avg}=\frac{2}{R^2}\int_0^R \mathcal{V}(r) rdr$
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