Multiple choice

Find out the wrong number in the following number series. 1 5 23 91 263 521 515

  1. 23

  2. 91

  3. 263

  4. 521

  5. 5
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The series follows the pattern: ×4-1, ×4+1, ×4-1 alternating. 1×4-1=3 (not 5), 5×4+1=21 (not 23), 23×4-1=91 (correct), 91×4+1=365 (not 263), 263×4-1=1051 (not 521). Actually the pattern is: subtract powers of 4: 4^0-1=0, 4^1-1=3, 4^2-1=15, 4^3-1=63, 4^4-1=255, 4^5-1=1023. Testing shows 91 breaks the pattern.

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