Find the wrong number in given series:89, 78, 86, 80, 85, 82, 83

The correct answer is Option (2) → 86

The terms go down and up in turn, so the differences are checked. They should be −11, +9, −7, +5, −3, +1: the odd numbers from 11 downwards, subtracted and added in turn.

• \(89-11=78\)

• \(78+9=87\), but the series has 86

• \(87-7=80\)

• \(80+5=85\)

• \(85-3=82\)

• \(82+1=83\)

Every later term fits only if the third term is 87; with 86, the next difference would be −6 and the pattern breaks. So 86 is the wrong number.