Figure shows Zener diode with a breakdown voltage of 40V connected to 100V DC source with a series resistance $R_S$ and a load resistor  $R_L$ . Find the output voltage across $R_S$ . 

Break down voltage of zener diode,  $V_2=40V$
input voltage,  $V_1=100V$
As the diode is reverse biased, it will operate in the break down region. Hence output voltage across $R_L$ is  $V_L=V_Z=40V$
Voltage across  $R_s = 100 -40 = 60 V$