Problem 7 · 2022 Math Kangaroo
Medium
Spatial & Visual Reasoning
symmetry
Otto fastens his licence plate to the car upside down, but it doesn’t matter because the plate looks exactly the same that way. Which of these plates could be Otto’s?
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Answer: B — 60 SOS 09
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Hint 1 of 2
Turning the plate upside down rotates it 180 degrees; which digits still read as valid digits after that?
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Hint 2 of 2
Only 0, 1, 8 (and the pair 6/9 swapping) survive a 180 turn; the whole string must read the same.
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Approach: test each plate for 180-degree symmetry
- Rotating 180 degrees, 0 stays 0, 1 stays 1, 8 stays 8, while 6 and 9 swap.
- The string must read identically after flipping and reversing order.
- Only 60 SOS 09 reads the same upside down, so the answer is B.
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