Understanding CMOS Circuit Behaviour with Resistive Loads using Thevenin theorem
Thevenin theorem states the equivalence between a "box" and a voltage-source/resistor series:







simulate this circuit &ndash; Schematic created using CircuitLab

In this case, the box is the dashed one on the right hand side:



The open circuit voltage of this box is 3.33V and the resistance with shorted generators is 667ohms.