You want to drive a red light-emitting diode with a voltage source of $U_{\rm s} = 5.0 ~\rm V$ and a current of $I_{\rm D} = 20 ~\rm mA$. You have found for red LEDs a voltage drop of $U_{\rm D} = 2.3 ~\rm V$ based on a web search. Now you want to know which resistor value from the E12 series (available to you) is correct as a shunt resistor.