R1两端电压U1=R1*E(R1+R2)=3E/(3+6)=E/3
R3两端电压U3=R3*E/(R3+R4)=1*E/(1+4)=E/5
Uab=U1-U3=E/3 -E/5=(2/15)E=7
E=52.5v