Discussion
One term of this sequence has been replaced by a question mark. Identify the pattern, then choose the number that belongs at the question mark.
3, 9, 27, 81, ?
Answer: B
Correct answer: 243
The gaps are 6, 18, 54, which follow no constant pattern, so the rule lies in the terms rather than in the steps between them. Each term is three raised to the power of its position: position 1 gives 3 and position 4 gives 81. The next position is 5, which gives 243. As a check, the same rule applied to the earlier positions reproduces 3, 9, 27, 81 exactly, so it accounts for every term shown and not merely the last. A series whose terms are one less than a power of two has differences that are themselves powers of two.
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