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 12 MARBLES:

Given twelve marbles, identical in size, shape and color. Which single marble is heavier in weight than the others? You are supplied with a balance and must conclude your determination using the balance three times only.


Solution:

First, weigh all 12 marbles, six on each side of the scale (weighing #1). Whichever side is heavier, take those six marbles and weigh three on each side (weighing #2). Again, whichever side is heavier, take those three marbles, placing one to the side, and weighing the other two, one on each side of the scale (weighing #3). During this weighing, if one marble weighs heavier than the other, the answer is obvious, and so too, if they balance perfectly, then the marble you put to the side is the heavier marble!