The four resistors on the left form a balance bridge, and the four resistors on the right can be regarded as open circuit or short circuit; So Rab=20/2+60/2=40 euros.

Figure 2.2:

The five resistors in the upper right corner are also balance bridges, and the resistor in the middle can be regarded as an open circuit or a short circuit, which is equivalent to 6 ohms. Then it is connected in series with the lower resistor and in parallel with the left resistor: Rab = 6/(6+6) = 6 *12/(6+12) = 4 ohms.