Discussion

Insert the missing number 16, 33, 65, 131, 261, (....)

Answer: A

Each term is roughly double the previous, with the correction alternating between \( +1 \) and \( -1 \).

\[ 16 \times 2 + 1 = 33 \]

\[ 33 \times 2 - 1 = 65 \]

\[ 65 \times 2 + 1 = 131 \]

\[ 131 \times 2 - 1 = 261 \]

\[ 261 \times 2 + 1 = \mathbf{523} \]

Doubling alone would give 522, so the alternating sign is what separates 523 from the 521 on offer.

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