Discussion
How many odd numbers are there which are immediately preceded by a perfect square ?
Answer: D
There are 6 such odd numbers.
Treating each digit as a number, the perfect squares among single digits are 1, 4 and 9 (1 counts, since \(1 = 1^{2}\)). Reading the sequence 4 3 1 5 7 8 3 1 6 2 9 7 1 2 1 3 5 2 6 8 6 5 4 3 2 1 3 8 7, the odd digits that come straight after one of them are:
4-3, 1-5, 9-7, 1-3, 4-3 and 1-3, at positions 2, 4, 12, 16, 24 and 27.
That makes 6. Leaving out 1 as a perfect square would give only 3 (the pairs after 4 and 9), which is why the smaller options appear.
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