Hi,
Let´s assume you have 12V at the battery.
Then you get 3V at the voltage divider output.
At the voltage divider output there are two resistors. 30k and 10k. They act as if they are in parallel.
therefore R_s = 1 / ( 1/10k + 1/ 30k) = 7.5k
***
you can test this.
R = U / I.
U output = 3V
if you connect an amperemeter at the output to GND, then the voltage becomes zero. (at least close).
Now the current should show a value of about 0.4mA.
Now calcualte R_s: R_s = U / I = 3V / 0.4mA = 7.5kOhms
Klaus