Find the wrong number in the series 3, 20, 6, 17, 12, 15, 24, 11.

12Correct answerYour answer15Correct answerYour answer17Correct answerYour answer24Correct answerYour answer

Show the answer and explanation

CorrectNot quiteAnswer B

Correct answer: B

The terms rise and fall, so split them. Odd places: 3, 6, 12, 24 (doubling), which is consistent.
Even places: 20, 17, 15, 11. The rule is −3: $20, 17, 14, 11$ .
So 15 is wrong; it should be 14.

Worth noting

Looking at the whole series at once, nothing looks off. Splitting into the two interleaved series shows the break in the second one.

Ten questions from Number and Letter Series, at your own pace.

Ten questions from Number and Letter Series, at your own pace.

Explanation