$B=\frac{{{\mu }_{{}^\circ }}NI}{2R}\Rightarrow \frac{{{B}_{2}}}{{{B}_{1}}}=\frac{{{\mu }_{{}^\circ }}\frac{{{N}_{2}}{{I}_{2}}}{2{{R}_{2}}}}{{{\mu }_{{}^\circ }}\frac{{{N}_{1}}{{I}_{1}}}{2{{R}_{1}}}}\xrightarrow{{{I}_{1}}={{I}_{2}}}\frac{{{B}_{2}}}{{{B}_{1}}}=\frac{{{N}_{2}}}{{{N}_{1}}}\times \frac{{{R}_{1}}}{{{R}_{2}}}(1)$ $N=\frac{\ell }{2\pi R}\Rightarrow \frac{{{N}_{2}}}{{{N}_{1}}}=\frac{\frac{{{\ell }_{2}}}{2\pi {{R}_{2}}}}{\frac{{{\ell }_{1}}}{2\pi {{R}_{1}}}}=\frac{{{\ell }_{2}}}{{{\ell }_{1}}}\times \frac{{{R}_{1}}}{{{R}_{2}}}(2)$ $(1),(2)\Rightarrow \frac{{{B}_{2}}}{{{B}_{1}}}=\frac{{{\ell }_{2}}}{{{\ell }_{1}}}\times {{(\frac{{{R}_{1}}}{{{R}_{2}}})}^{2}}\xrightarrow[\frac{{{B}_{2}}}{{{B}_{1}}}=\frac{5}{2}]{{{R}_{2}}=2{{R}_{1}}}\frac{5}{2}=\frac{{{\ell }_{2}}}{{{\ell }_{1}}}\times \frac{1}{4}\Rightarrow \frac{{{\ell }_{2}}}{{{\ell }_{1}}}=10$