Answer
$B=467{\mu}T$
Work Step by Step
We know that,
$B=\frac{mv}{Rq}$
We plug in the known values in this formula to obtain:
$B=\frac{9.109\times 10^{-31}(2.05\times 10^7)}{0.25(1.6\times 10^{-19})}$
$B=466.8\times 10^{-6}$
$B\approx467\times 10^{-6}$
$B\approx467{\mu}T$
