Positive input terminal - Basic Circuit Analysis
To figure it out, consider


What's the current going in to an input pin of an ideal op-amp?
Therefore, according to Ohm's law, what's the voltage across that resistor?


Stop and work this out, then continue reading...


   The reason that resistor is there is because real op-amps aren't ideal. There will be a small current in or out of the input pin. To minimize the error caused by this current, you want an equal effective resistance connected to each of the input pins. So really the resistor value should be 20 || 100 = 16 or 17 kohms, not 10 kohms.