Hello,
The formula is this:
R1=(Vin-Vad)*R2/Vad
where
R1 is the upper resistive voltage divider resistor, and
R2 is the lower resistive voltage divider resistor, and
Vin is your input voltage (20v in this case), and
Vad is the max supply voltage for the PIC assuming you are using that as the AD reference also (5v in this case), and
R2 is a resistor picked according to the input resistance requirements of the PIC ADC, where 10k is a decent choice.
Given the above, your lower resistor would be 10k and your upper resistor would be 30k.
You might also note that depending on the accuracy and drift requirements you may want to go with a better regulator than the 7805 type.