What is the voltage of the output in the following circuit?
1) Op amp inputs are high-impedance. Therefore, the + input is at 5 volts.

2) Op amps exhibit very high gain. Therefore, if the op amp is being operated usefully, the - input is at 5 volts, since any output will produce a very small (usefully approximated as zero) voltage difference between + and - inputs.

3) The voltage across R1 will be 5 volts and the current through R1 will be 5 mA by Ohm's Law.

4) Op amp inputs are high-impedance. Therefore, the current through R2 will be 5 mA. 

5) The voltage across R2 will be 20 volts by Ohm's Law.

6) The output voltage will be the sum of the voltages across R1 and R2, or 25 volts.