Calculate op-amp input voltage with potential divider and series resistor
The resistance is "seen", but its effect is so small that unless you're working in precision circuitry you will not be able to measure it. (the op amp input is really high impedance)
You can refer to the bias input current if you want to know the voltage drop as this will dominate that input resistance in most op amps, e.g. 5pA * 20K = 100nV, 
As the effect is about 7 significant figures away from the the divider voltage, in this case your safe to ignore it, if you had say 1 Giga-ohm resistors for your divider then it would be a different story (noise dominates there, but the DC value would be affected)
refer to the input bias currents if you want the 20K drop, it will dominate the input impedance.