Simple example, same as in the book. First on the left we have two 1K Ohm resistors in series. R=R1+R2=1k+1k=2k Ohm. I=9V/2k=4.5mA. Then on the right we have two 1k resistors in parallel. Here R=1/(1/R1 + 1/R2)=1/(1/1000+1/1000)=500. So I = 9/500=18mA in the circuit (9mA in each leg of the circuit with the resistor).
