\frac{B}{B_1} = \frac{v^2}{\left(\frac{3}{4}v\right)^2} \\
\frac{400}{B_1} = \frac{1}{\frac{9}{16}} \\
\frac{400}{B_1} = \frac{16}{9} \\
16 B_1 = 3600 \\
B_1 = 225\,cm^2