Discussion
One term of this sequence has been replaced by a question mark. Settle on the rule the terms obey and give the next number.
81, 100, 121, 144, ?
Answer: B
Correct answer: 169
The gaps are 19, 21, 23, which follow no constant pattern, so the rule lies in the terms rather than in the steps between them. Each term is the square of a whole number, the numbers running consecutively: position 9 gives 81 and position 12 gives 144. The next position is 13, which gives 169. As a check, the same rule applied to the earlier positions reproduces 81, 100, 121, 144 exactly, so it accounts for every term shown and not merely the last. Growth that accelerates points to multiplication; growth that is steady points to addition.
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