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Find out the wrong number in the series 1, 3, 10, 21, 64, 129, 356, 777

Answer: D

The multiplier alternates between 2 and 3, with 1 added each time.

\[ 1 \times 2 + 1 = 3 \quad\checkmark \qquad 3 \times 3 + 1 = 10 \quad\checkmark \]

\[ 10 \times 2 + 1 = 21 \quad\checkmark \qquad 21 \times 3 + 1 = 64 \quad\checkmark \]

\[ 64 \times 2 + 1 = 129 \quad\checkmark \qquad 129 \times 3 + 1 = 388 \quad\text{not } 356 \]

So 356 is wrong. The final term settles it: \( 388 \times 2 + 1 = 777 \).

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