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Refer to the following series and answer the question. (All numbers are single-digit numbers only. Counting to be done from left to right.) (Left) 4 3 8 2 5 6 9 4 7 2 8 1 3 4 6 5 9 2 8 7 3 4 (Right) How many such odd digits are there, each of which is immediately preceded by an even digit and immediately followed by an even digit?
2
3
4
5
4
Scan the sequence for odd digits first, then instantly check the immediate left and right digits for even numbers.
Scan the sequence for odd digits first, then instantly check the immediate left and right digits for even numbers.
Analyze the series structure
The given sequence is 4, 3, 8, 2, 5, 6, 9, 4, 7, 2, 8, 1, 3, 4, 6, 5, 9, 2, 8, 7, 3, 4. We must identify odd numbers (3, 5, 9, 7, 1, 3, 5, 9, 7, 3) surrounded by even numbers on both sides.
Evaluate triplets (Even - Odd - Even)
1: (4, 3, 8) - Found: 3 is odd, 4 and 8 are even. 2: (8, 2, 5) - Ignore: 2 is even. 3: (2, 5, 6) - Found: 5 is odd, 2 and 6 are even. 4: (6, 9, 4) - Found: 9 is odd, 6 and 4 are even. 5: (4, 7, 2) - Found: 7 is odd, 4 and 2 are even. 6: (8, 1, 3) - Ignore: 1 is odd but 3 is odd. 7: (3, 4, 6) - Ignore: 3 is odd. 8: (6, 5, 9) - Ignore: 9 is odd. 9: (9, 2, 8) - Ignore: 9 is odd. 10: (8, 7, 3) - Ignore: 3 is odd.
Verify final count
Reviewing the valid triplets: (4, 3, 8), (2, 5, 6), (6, 9, 4), and (4, 7, 2). Wait, let's re-verify: 4-3-8 is valid. 2-5-6 is valid. 6-9-4 is valid. 4-7-2 is valid. Total is 4.
A: Incorrect because it underestimates the valid triplets; B: Incorrect because it underestimates the valid triplets; D: Incorrect because it overestimates the count by including non-matching triplets.
C is correct because the triplets 4-3-8, 2-5-6, 6-9-4, and 4-7-2 all satisfy the condition of an odd digit being flanked by even digits on both sides, totaling 4 instances.
When counting specific patterns in a long string, mark the sequence on your scratchpad to avoid double-counting or missing overlaps.
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