Blue: If all resistors are equal, the final output voltage is the (negative) sum of all input voltages.
Green: If the feedback resistor value is equal to the total parallel input resistance, the output voltage is the (negative) average of all input voltages.
If the input resistors values are each different, each input voltage will have a different weight on the output average voltage, given by (Rf/Rx)Vi.
So, the bigger the value of an input resistor the less weight that input voltage will have on the output average voltage, and the smaller the value of an input resistor the more weight that input voltage will have in the average output voltage.
