\(=>R4//\left[R1nt\left(R2//R3\right)\right]\)
\(=>U4=U123=U=120V\)
\(=>I4=\dfrac{U4}{R4}=\dfrac{120}{20}=6A\)
\(=>I123=\dfrac{U123}{R123}=\dfrac{120}{R1+\dfrac{R2.R3}{R2+R3}}=\dfrac{120}{20+\dfrac{20.20}{20+20}}=4A=I23\)
\(=>U23=I23.R23=4.\dfrac{20.20}{20+20}=40V=U3\)
\(=>I3=\dfrac{U3}{R3}=\dfrac{40}{20}=2A\)
\(=>Ia=I4+I3=2+6=8A\)