CombinatoricsDifficulty 4.9AIMEProve itUnited States
Problem:
There are 10 bags full of coins. All coins look the same and all weigh 10 grams, except the coins from one bag that are fake and all weigh 9 grams. Given a scale, how could you tell which bag has the wrong coins in just one measurement? Explain your answer!
Solution
Solution:
Take one coin from the first bag, 2 from the second, 3 from the third, ..., 10 from the 10th and place them on the scale. The total mass shown by the scale should be (1+2+⋯+10)⋅10−x grams, where x is the number of fake coins (each coin weighs 10 grams, except for the fake ones that are one gram lighter). Since the scale will show 550−x grams, we can see how many coins are fake and that is enough to deduce from which bag those coins were taken.
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Source: MathNet,
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