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Separate the rocks into 3 equal piles of 4. Weigh each, the odd one out contains the different rock (rock X). This also will tell you if it is lighter or heavier and the weight of each individual other rock (Y).
Take two rocks from the group you now know contains rock X and weigh them. If they weigh 2Y, take one of the two remaining rocks and weigh it. It is either X or a Y.
If they don't weigh 2Y, pick one and weigh. It is either X or a Y.
Obviously if the rock weighs Y, the other rock is X and vice versa.
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Wait wait wait... I want to clarify something.... What kind of scale are we using? Are we using something that weighs the thing you put in it? Or are we talking about a scale like this...
