Multiple choice

Find the odd term out of the series: 1, 2, 6, 15, 31, 56, 91

  1. 15

  2. 31

  3. 56

  4. 91

  5. None - all are correct
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Differences: 2-1=1, 6-2=4, 15-6=9, 31-15=16, 56-31=25, 91-56=35. The differences are squares (1^2, 2^2, 3^2, 4^2, 5^2). The next difference should be 6^2=36. 56+36=92, not 91.

AI explanation

The pattern is to add consecutive square numbers starting with 1 to the previous term. So, 1 plus 1 squared is 2, 2 plus 2 squared is 6, 6 plus 3 squared is 15, 15 plus 4 squared is 31, and 31 plus 5 squared is 56. Following this logic, 56 plus 6 squared equals 92, which makes 91 the odd term out.

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