We must calculate the equivalent resistance of the circuit that is highlighted in yellow in the Figure 02-17.2. Note that we have two resistors in parallel: a resistor of 12 ohms and another one of 24 ohms. Doing the calculations yields 8 ohms. In the circuit highlighted in green, we also have two resistors in parallel: one of 60 ohms and one of 20 ohms. Calculating, results 15 ohms.

In the Figure 02-17.3, we can see how the circuit stayed after the resistor parallel was made. Now let's calculate the equivalent resistance of the circuit highlighted in yellow. Notice that the three resistors are in series. So just add their values and we find 30 ohms. So we reduce to a resistor of 30 ohms, which in turn is in parallel with the resistor of 120 ohms. Calculating the parallel, we find the value of 24 ohms.

Now, in the Figure 02-17.4, we see the final circuit greatly reduced. Notice how easy it was to calculate the total resistance seen by the a-b terminals. Just add them up, because they are all in series. And so, we find the final result making some associations that simplified the circuit, allowing us to calculate the equivalent resistance of the circuit that initially presented as hard to understand.