Hi,
From the drawing provided by WTP Pepper the voltage at the junction of R2 and R1 with the diode open circuited is:
Vo=V1*R1/(R1+R2)
With the diode voltage drop being Vd, the diode conducts if the following condition is met:
Vo>=Vd
If that condition is not met then the voltage at the junction of R2 and R1 is Vo, but if that condition is met then the voltage at the junction is equal to the diode voltage drop Vd.
Since when V1 is 20 volts that condition is met, the voltage across the diode is equal to Vd which here is 2 volts.
With that voltage being 2 volts, the current through R2 is:
iR2=(V1-Vd)/R2
where here is:
iR2=(20-2)/R2
The current through R1 is:
iR1=Vd/R1
and so the current through the diode is:
iD=iR2-iR1
That should allow you to calculate whatever you need for this circuit.