\(f _{2}=\left(\frac{330+ v _{0}}{330}\right)\left(\frac{330}{330- v _{ B }}\right) 420\)
\(490=\left(\frac{330+ v _{ B }}{330- v _{ B }}\right) 420\)
\(\frac{7}{6}=\frac{330+ v _{ B }}{330- v _{ B }}\)
\(v _{ B }=\frac{330}{13} m / s\)
\(=\frac{330}{13} \times \frac{18}{5} \approx 91 km / hr\)