Semiconductor Electronics Part 3

The following diagram shows a Zener diode as a voltage regulator. The Zener diode is rated at Vz=5V and the desired current in load is 5 mA . The unregulated voltage source can supply upto 25 V . Considering the Zener diode can withstand four times of the load current, the value of resistor Rs (shown in circuit) should be ____ Ω.