Answer
$7.0\mu m$
Work Step by Step
We know that
$r=\frac{\mu_{\circ}}{2\pi B}$
We plug in the known values to obtain:
$r=\frac{(4\pi \times 10^{-7})(3.5)}{2\pi (0.10)}$
$r=7.0\times 10^{-6}=7.0\mu m$
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