In the series given below, one number does not fit. Find that number.
3, 5, 11, 31, 83, 245, 731
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 31
The terms grow about three times each step, so compare each term with three times the one before. The rule is multiply by 3 and subtract 4:
\(3\times3-4=5\)
\(5\times3-4=11\)
\(11\times3-4=29\), but the series has 31
\(29\times3-4=83\), which matches the next term
\(83\times3-4=245\)
\(245\times3-4=731\)
The term after it fits only if the fourth term is 29; with 31 it would be \(31\times3-4=89\). So 31 does not fit the series.