$B=n\left[\frac{\mu_{0} i}{2 R}\right]=n\left[\frac{\mu_{0} i}{2 \frac{L}{2 \pi n}}\right]$
$B=n\left[\frac{\mu_{0} i}{2 L} \times 2 \pi n\right]$
$=\frac{\mu_{0} i}{2 L \times 2 \pi} \times 2 \pi n^{2} \times 2 \pi$
$B=\frac{\mu_{0} i}{4 \pi} \frac{4 \pi^{2} n^{2}}{L}$
