U1=I1×R1=0.2A×20Ω=4V
I2=U/R2=4V/30Ω=0.13A
I总=I 1+I 2=0.2A+0.13A=0.33A
R总= R1×R2/(R1+R2)=20Ω×30Ω/(20Ω+30Ω)=12Ω