如图所示，R1=20欧，当开关闭合时电流表示数为0.3A，当开关S断开时，电流表的示数变化了0.1

断开后电流为：I2=0.3-0.1=0.2AU=I2*R1=0.2*20=4V闭合时电流：I1=0.3AR并=U/I1= 断开后电流为：I2=0.3-0.1=0.2AU=I2*R1=0.2*20=4V闭合时电流：I1=0.3AR并=U/I1=4/0.31/R并=0.3/腔猛州41/R并=1/R1+1/伍蔽R20.3/4=1/20+1/知渣R21/R2=0.3/4-1/20=0.5/20=1/40R2=40Ω