Answer
$B=1.5 \times 10^2 \mathrm{~T}$.
Work Step by Step
From Fig. $32-14$ we estimate a slope of $B / T=0.50 \mathrm{~T} / \mathrm{K}$ when $M / M_{\max }=50 \%$.
So $B=0.50 \mathrm{~T}\\=(0.50 \mathrm{~T} / \mathrm{K})(300 \mathrm{~K})=1.5 \times 10^2 \mathrm{~T}$.
