电源电压不变,电阻R1=20欧,R2=30欧,当开关S1,S2均断开时,电压表V的示数为6V,电流

（1）s1.s2断开时,V与R3并联
R3=U/I=6V/0.6A=10欧
R总=R1+R3=30欧
U总=I*R总=18V
（2）S1.S2闭合时,R1.R2并联 V.R3被短路
V=0
R总=（R1* R2）/(R1+R2)=12欧
I总=U总/R总=1.5A