\(\frac{{{U}_{1}}}{{{U}_{2}}}=\frac{{{N}_{1}}}{{{N}_{2}}}=\frac{{{U}_{1}}}{20}->\frac{{{U}_{1}}}{{{N}_{1}}}=\frac{20}{{{N}_{2}}}\)
\(\frac{{{U}_{1}}}{{{U}_{2}}'}=\frac{{{N}_{1}}}{{{N}_{2}+60}}=\frac{{{U}_{1}}}{25}->\frac{{{1}_{}}}{{{N}_{2}+60}}=\frac{20}{{{25.N}_{2}}}->{{{N}_{2}}}=240 vòng\)
\(\frac{{{U}_{1}}}{{{U}_{2}}''}=\frac{{{N}_{1}}}{{{N}_{2}-90}}->{{{U}_{2}}''}=\frac{{{20}_{}}}{{{240}_{}}}.(240-90) =12,5 V\)