One of the lessons I learned from my high school electronics teacher was DC analysis, or how to combine multiple resistors to obtain the value you needed.
He would stand in the door way and hand out a quiz as you entered the room, this quiz sheet had a few resistors in come series/parallel combo, your task was to solve for the equivalent resistance, voltage drop, current, and power dissipation , each day there would be one more resistor then yesterdays quiz. This lesson was used in lab, if a circuit needed a 10k resistor, there were never enough 10k's for everyone to have, so you had to build your resistor from what was available. I never knew if that was by design or not.
The other day, while setting up a test environment for one of our building automation controllers, we needed a 500 ohm resistor to test the range of one the universal inputs (voltage, current or resistance).
Problem, we needed 500 ohms, we only had available that day a hand full of 120 ohm resistors, I said it was possible, all I got were stares from the other engineers/techs in the room.
Can you solve the problem? Build a 500 ohm resistor using some number of 120 ohm resistors.
How many does it take?