10K+20K=30K. 20K divided by 30K= .66. next 5Vx.66=3.3V ok I understand your equation. however I can not get my results to check out correctly. So here we go ballast = 4520 ohms resistor = 660 ohms rt = 5180 660/5180= .1274 5v x .1274 = .637 volt now when I tes... Login to read more.