Finding the heavier ball | Summary and Q&A

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July 15, 2014
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Khan Academy
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Finding the heavier ball

TL;DR

Using a scale, determine the heaviest ball out of nine identical balls in a minimum number of weighings.

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Questions & Answers

Q: How many weighings are needed to find the heavier ball out of nine identical balls?

The minimum number of weighings required is two, as explained in the content.

Q: How does the first weighing help in narrowing down the heavier ball?

The first weighing, with three balls on each side, helps determine if the heavier ball is in the group of three or not.

Q: What should be done after a balanced first weighing?

If the first weighing is balanced, one ball from the group of three is weighed against another ball to identify the heavier one.

Q: What if the first weighing shows an imbalance?

In case of an imbalance in the first weighing, further weighings are done with the heavier side to narrow down the heaviest ball.

Summary & Key Takeaways

  • There are nine identical balls, one of which is slightly heavier.

  • A scale is the only instrument available to determine the heavier ball.

  • By weighing groups of balls and ruling out 2/3 each time, the heavier ball can be identified in two weighings.

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