Hey Guji! Its really a difficult question..u need to do either lot of thinking or lot of surfing... I think this question is way high upto the reach of many people here... I could understand the proof but its no point copy pasting in this thread. This was asked in Virginia Tech Regional Mathematics Contest 98. And by the way ... no body gave a mathematical proof to my question of 4 stones... it was 1 3 9 and 27? but how? mathematically? Guji..u sud try that out. cheers!