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This question already has an answer here:

I have 9 coins that look identical, but one is heavier than the others. I want to identify that one.

My brother has an old-fashioned balance scale that I can use, but I'll have to do annoying chores for him for every time I use that scale (that is, every time I use it to compare the weights of two (sets of) coins). Can you help me find the heavier coin using the scale as little as possible?

• The scale can handle the weight of all coins together, so there is no restriction in that regard.
• Please give not only the number of times you need to weigh, but also explain what you weigh and why.

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marked as duplicate by phenomist, Wen1now, Oray, Deusovi May 30 '18 at 7:46

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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I am pretty sure this is asked before but I will give you the shortest answer possible:

Divide coins into three, A=3,B=3,C=3, weight AvB, if they are equal, C has the heaviest one, if A weighs more, A has, otherwise B has.

Then

Divide Heavier sets of coins again, 1v1, leaving 1 outside, if it balances the one outsite is heavier, or whichever weighs more is heavier one.

so

Two weighing is optimal solution.

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