You have 9 identical-looking steel spheres. Eight of them weigh 10 grams each, but one is a manufacturing defect weighing 11 grams. Using a balance scale only twice, how can you guarantee finding the heavy sphere?
Divide the spheres into three groups of 3 (A, B, C). Weigh Group A against Group B.
If Group A and B balance, the heavy sphere is in Group C; if one group is heavier, the heavy sphere is in that group. From that group of 3, weigh 1 sphere against 1 sphere. If they balance, the unweighed 3rd sphere is the heavy one; otherwise, the heavier side holds it.
About this riddle
Logic riddles give you a few facts and ask what must be true. List what you know, cross out what cannot be, and see what is left.
Difficulty: Hard. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
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