\(MCD:R_1nt\left[\left(R_2//R_3\right)//R_4\right]\)
\(\Rightarrow R_{td}=R_1+R_{234}=5+\dfrac{\left(\dfrac{12\cdot8}{12+8}\right)\cdot20}{\left(\dfrac{12\cdot8}{12+8}\right)+20}=\dfrac{275}{31}\Omega\)
\(\Rightarrow I=I_1=I_{234}=\dfrac{U}{R}=\dfrac{30}{\dfrac{275}{31}}=\dfrac{186}{55}A\)
\(\Rightarrow U_1=I_1\cdot R_1=\dfrac{186}{55}\cdot5=\dfrac{186}{11}V\)
\(\Rightarrow U_{234}=U_{23}=U_2=U_3=U_4=U-U_1=30-\dfrac{186}{11}=\dfrac{144}{11}V\)
\(\Rightarrow\left\{{}\begin{matrix}I_2=\dfrac{U_2}{R_2}=\dfrac{\dfrac{144}{11}}{12}=\dfrac{12}{11}A\\I_3=\dfrac{U_3}{R_3}=\dfrac{\dfrac{144}{11}}{8}=\dfrac{18}{11}A\\I_4=\dfrac{U_4}{R_4}=\dfrac{\dfrac{144}{11}}{20}=\dfrac{36}{55}A\end{matrix}\right.\)