Hi! I can't find the method of the solution to this question, it was an exercise on an exam.. There are two grounding electrodes buried in the ground. We make three measurments. First, we measure the first electrode's grounding resistance, which is: 6 ohms. Then, the second's: 7 ohms. Then the resistance between the two electrodes: 5 ohms. The question is, if we connect the two electrodes with an ideal wire (which has no resistance), then what will be the system's grounding resistance. We only know this much, nothing more. The correct answer is: 5.2 ohms. How can you acquire this result? 3.23 ohms is not the right answer... Thanks.