Maths Olympiad Prep

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Combinatorics Difficulty 4.9 AIME Prove it United 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)10x(1+2+\cdots+10) \cdot 10 - x grams, where xx 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 550x550 - 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, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.