Problem 6 · 2023 Math Kangaroo
Medium
Spatial & Visual Reasoning
path-tracingwork-backward
Four ribbons M, N, P and Q are wrapped around a box, one after another (see picture). In what order were they wrapped around the box?
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Answer: D — N, M, Q, P
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Hint 1 of 2
Wherever two ribbons cross, the one wrapped later lies on top of the one wrapped earlier.
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Hint 2 of 2
Find the ribbon that is on top everywhere (it was last), then peel back through the crossings to read the order.
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Approach: use the over/under crossings to order the wrappings from last to first
- At every crossing the upper ribbon was wrapped after the lower one.
- Reading the crossings, P lies over the others, so it was last; beneath it comes Q, then M, and N is covered by all of them, so it was first.
- Putting them from first to last gives N, M, Q, P.
- The order is D.
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