Hi friends Problem is - https://fbcdn-sphotos-b-a.akamaihd.net/hphotos-ak-ash3/s480x480/548739_2677144185130_1329975308_n.jpg Attempt - Well the potential at the origin is 10V. But if we move a distance of 1unit from the origin in any axis, the potential becomes 8 V. The difference is only 2V. The potential is decreasing as we move away from the origin. And the point (1,1,1) is not too far from point (1,0,0). Hence the potential decreases by a small amount It'll not be zero because otherwise the difference would be of 8 V, which is unsatisfactory. Hence the answer would be 4 V. And the answer is also correct according to the book. But friends the problem is that I am unable to make any mathematical explanation for this question. Please fiends give your suggestions. Thank you all in advance.