题目貌似有点混乱……
若S能短路R1,则S必能短路R2,因为R1,R2两端等势.
猜测:S能断路R1,S闭合干路1.5A,S断开1A.
S断开时电源电压U=1A*R2=R2
闭合时U=R2=1.5*R1R2/(R1+R2)
解得R2=6Ω