In following questions, one term in number series is incorrect. - Find out the incorrect number 89, 78, 86, 80, 85, 82, 83

Answer: C. 86

The sequence is - 11, + 9, - 7, + 5, - 3, + 1.
So, 86 is wrong and should be replaced by (78 + 9)  i.e. 87.