Discussion
How many 6's are there in the following number series, each of which is immediately preceded by 1 or 5 and immediately followed by 3 or 9 ?
2 6 3 7 5 6 4 2 9 6 1 3 4 1 6 3 9 1 5 6 9 2 3 1 6 5 4 3 2 1 9 6 7 1 6 3
Answer: C
There are three such 6's.
Go through every 6 in the series and check its neighbours:
- 2 6 3, 5 6 4 and 9 6 1: the 6 is preceded by 2 or 9, or followed by 4, so these fail.
- 1 6 3 (the 15th digit): preceded by 1, followed by 3, so it counts.
- 5 6 9 (the 20th digit): preceded by 5, followed by 9, so it counts.
- 1 6 5 and 9 6 7: followed by 5, or preceded by 9, so these fail.
- 1 6 3 at the end: preceded by 1, followed by 3, so it counts.
That gives three. Answers of one or two miss one of the qualifying 6's, and four counts one that fails a condition.
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