Find the term which doesn't fit into the series given below:
H4Q, K10N, N20K, Q43H, T90E
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → K10N
In a letter-number series, each part of the term follows its own rule, so the first letters, the numbers and the last letters are checked separately.
First letters H (8), K (11), N (14), Q (17), T (20): each is 3 places ahead in the alphabet
Last letters Q (17), N (14), K (11), H (8), E (5): each is 3 places back
Both letter rules hold in every term, so the odd term must be found from the numbers 4, 10, 20, 43, 90. They follow “double and add 1, 2, 3, 4 in turn”:
\(4\times2+1=9\), but the series has 10
\(9\times2+2=20\)
\(20\times2+3=43\)
\(43\times2+4=90\)
The later terms fit only if the second number is 9; with 10, the next number would be \(10\times2+2=22\), not 20. So the term should be K9N, and K10N does not fit.