(1)
设原来电阻是R
I=U/R
I1=U/(R+4)
I/I1=1/2
R/(R+4)=1/2
R=4
(2)
设串联电阻是r
I=U/4
I2=U/(r+4+4)
I/I2=1/5
4/(r+8)=1/5
r=12