In the series given below, one number does not fit. Find that number.
2, 5, 12, 27, 57, 121, 248
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 57
The terms roughly double each step, so compare each term with twice the one before. The rule is multiply by 2 and add 1, 2, 3, 4, ... in turn:
\(2\times2+1=5\)
\(5\times2+2=12\)
\(12\times2+3=27\)
\(27\times2+4=58\), but the series has 57
\(58\times2+5=121\), which matches the next term
\(121\times2+6=248\), the last term
The term after it fits only if the fifth term is 58; with 57 it would be \(57\times2+5=119\). So 57 does not fit the series.