The wrong number in the given series is:
7, 28, 63, 124, 215, 342, 511
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 28
The terms lie just below the cubes 8, 27, 64, 125, …, so each term is one less than a cube, \(n^3-1\), for \(n=2, 3, 4, \ldots\)
\(2^3-1=7\)
\(3^3-1=26\), but the series has 28
\(4^3-1=63\)
\(5^3-1=124\)
\(6^3-1=215\)
\(7^3-1=342\)
\(8^3-1=511\)