R₁ = R₂ = R₄ = 10Ω
R₃ = 40Ω
R(ech) = ?
[tex]\frac{1}{R_e_{1}}=\frac{1}{R_2}+\frac{1}{R_3}=\frac{1}{10}+\frac{1}{40}=\frac{50}{400}=\frac{5}{40}\implies R_{e_{1}}=\frac{40}{5}=8\ {\Omega}[/tex]
[tex]R_{e_{2}}=R_1+R_{e_{1}}+R_4=10+8+10=28\ {\Omega}[/tex]
