\(B_{1}=\frac{N_{1} \mu_{0} i}{2 R_{1}}\)

For \(N _{2}=5\)

Radius of coil \(= R _{2}=\frac{ N _{1} \times R _{1}}{ N _{2}}\)

\(B _{2}=\frac{ N _{2} \mu_{0} i }{ R _{2}}\)

\(\frac{ B _{2}}{ B _{1}}=\frac{ N _{2}}{ N _{1}} \frac{ R _{1}}{ R _{2}}=\frac{ N _{2}}{ N _{1}} \times \frac{ N _{2}}{ N _{1}} ; \quad \frac{ B _{2}}{ B _{1}}=\frac{25}{4}\)