Hence, for equal current in load and Zener diode, RL=RZ=440 Ω
Now, total resistance will be: 4402+RS=RS+220
Since current through Zener diode is 0.25A, current through combination will be: 0.25 A×2=0.5 A.
Hence, for current to be maximum, the maximum voltage can be 180 V.
Therefore, 180=0.5(RS+220)
RS=140 Ω