$U=\frac{1}{2}L{{I}^{2}}\Rightarrow 0/1=\frac{1}{2}\times L\times {{20}^{2}}\Rightarrow L=5\times {{10}^{-4}}H$ $L={{\mu }_{{}^\circ }}\frac{A{{N}^{2}}}{\ell }\Rightarrow 5\times {{10}^{-4}}=\frac{4\times 3\times {{10}^{-7}}\times 5\times {{10}^{-4}}\times {{N}^{2}}}{30\times {{10}^{-2}}}\Rightarrow 1=4\times {{10}^{-6}}\times {{N}^{2}}$ $\Rightarrow {{N}^{2}}=\frac{1}{4\times {{10}^{-6}}}=0/25\times {{10}^{6}}=25\times {{10}^{4}}\Rightarrow N=5\times {{10}^{2}}=500$