Problem 10 · 2010 Math Kangaroo
Medium
Spatial & Visual Reasoning
path-tracing
In the following figures you see five elastic bands, but only one of them is tied in a real knot. Which one?
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Answer: D
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Hint 1 of 2
Pretend you grab each band by two ends and pull it straight.
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Hint 2 of 2
Four bands fall open into a plain loop; only a real knot stays tangled.
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Approach: imagine pulling each band straight
- Trace one band at a time, following each strand as it goes over and under.
- Four of the bands are only crossed loops that open up flat when you pull them.
- The one that stays knotted no matter how you pull is band D.
Mark:
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