In each of these questions a number series is given. In each series, only one number is wrong. Find out the wrong number. 2 13 27 113 561 3369 23581

This series follows: 2×5+3=13, 13×2+1=27, 27×4+5=113, 113×5-4=561, 561×6-... The pattern involves multiplying by increasing numbers and adding/subtracting. Another approach: 2 to 13: +11, 13 to 27: +14, 27 to 113: +86, 113 to 561: +448, 561 to 3369: +2808. Try: 2×6+1=13, 13×2+1=27 (should be 13×3-12?), 27×4+5=113. The number 13 seems wrong - it should be 2×7-1=13? Actually: 2×6+1=13, 13×2+1=27. Pattern: ×6+1, then ×2+1 doesn't fit. If 2→something→27→113, then 13 might be wrong. Let's try: 2×7-1=13 (fits), 13×3-12=27, 27×4+5=113, 113×5-4=561. If 13 were replaced by say 15: 2×8-1=15, 15×2-3=27. The answer identifies 13 as wrong.