Electrons and Resistance help?

I was wondering if there exist an equation which I could use to compute for the voltage/current at the other end of the resistor given the voltage/current at the other end. I need to compute the exact voltage/current at the end independent of the whole network. Example, consider a 100-ohm resistor connected in series to parallel resistors 20-ohm and 40-ohm. How can I get the voltage/current at the 20-ohm resistor without considering the others? Likewise how can it be done on the 100-ohm. I don't really need the specific resistor any resistor in the said network will be fine. P.S. I have tried ohm's law, if there is a better way, I would really appreciate it. Quantum mechanics, particle physics formulas would also help (as well as links). Thanks in advance ☺