[tex]R_s=R_1+R_2=4+5=9\Omega\\U_1=R_1I_1=>I_1=\frac{U_1}{R_1}=\frac{8}{4}=2A=>I_1=2A\\ I_1=I_2=I=>I=2A\\U_2=R_2I_2=R_2I=5*2=10V=>U_2=10 V\\I=\frac{E}{R_s+r}<=>IR_s+Ir=E=>r=\frac{-IR_s+E}{I}=\frac{-2*9+20}{2}=\frac{2}{2}=1\Omega=>r=1\Omega\\[/tex]
[tex]u=rI=1*2=2V\\E=U+u=>U=E-u=20-2=18 V=>U=18 V\\\frac{U}{u}=\frac{18}{2}=9[/tex]
