Multiple choice

In each of the following number-series only one number is wrong. Find out that wrong number. $3 21 78 326 1648 9902$

  1. $78$
  2. $326$
  3. $3$
  4. $1648$
  5. $21$
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E Correct answer
Explanation

The pattern is: multiply by 6 and subtract 3, then multiply by 4 and subtract 6, then multiply by 5 and subtract 4, repeating with variations. Actually it's simpler: ×6-3, ×4-6, ×5-4... 21 is wrong because 3×6-3=15 should be second term. The actual pattern is ×7, ×3+15, ×4+14, ×5+12... 21 doesn't fit.

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