Which set of letters when sequentially placed at the gaps in the given letter series shall complete it?
_BA_BBA_AB_B
Answer: B
Correct answer: AAAB
Filling the gaps gives \[ \mathrm{A\,B\ AA\,BB\ AAA\,BBB}, \] which is one A and one B, then two of each, then three of each. The pattern is invisible until the blanks are filled, so the method is to try each option and look for structure rather than to guess at the rule first.
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