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Refer to the following series and answer the question (All numbers are single-digit numbers only. Counting is to be done from left to right.) (Left) 2 8 1 3 5 2 6 7 3 4 9 6 7 8 4 1 3 4 6 (Right) How many such even digits are there each of which is immediately preceded by an odd digit and immediately followed by an even digit?
Four
Two
Three
One
Three
Scan for even digits first, then check the immediate neighbors: Odd-Even-Even (O-E-E).
Scan for even digits first, then check the immediate neighbors: Odd-Even-Even (O-E-E).
Analyze the series
The given series is: 2 8 1 3 5 2 6 7 3 4 9 6 7 8 4 1 3 4 6. The requirement is to find even digits (2, 4, 6, 8) preceded by an odd digit (1, 3, 5, 7, 9) and followed by an even digit.
Identifying the patterns
We look for the pattern O-E-E: 1) 5-2-6 (Odd 5, Even 2, Even 6) - Count 1. 2) 3-4-9 (Wait, 9 is odd, skip). 3) 9-6-7 (Wait, 7 is odd, skip). 4) 7-8-4 (Odd 7, Even 8, Even 4) - Count 2. 5) 1-3-4 (Wait, 3 is odd, skip). 6) 3-4-6 (Odd 3, Even 4, Even 6) - Count 3.
Final count verification
The three triplets meeting the criteria (Odd-Even-Even) are 5-2-6, 7-8-4, and 3-4-6. Therefore, there are exactly three such even digits.
A: Four is incorrect as only three instances satisfy the condition. B: Two is incorrect as it misses the third valid instance found in the sequence. D: One is incorrect as it fails to identify the three distinct occurrences of the required pattern.
C is correct because the series contains exactly three even digits (2, 8, 4) that are immediately preceded by an odd digit and followed by an even digit (5-2-6, 7-8-4, and 3-4-6).
When scanning long sequences, write down the numbers and use small markers or brackets for the identified triplets to avoid skipping digits or miscounting due to eye fatigue.
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