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4, 5, 11, 34, ?, 686 — what replaces the question mark?

Competition practiceNumber Series

A136
B140
C137
D138

Step-by-Step Solution

The pattern is

\times 1 + 1, \times 2 + 1, \times 3 + 1, \times 4 + 1 : 34 \times 4 + 1 = 137

(and

137 \times 5 + 1 = 686

).

Answer: C) 137

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