Find the term which doesn't fit into the series given below:

H4Q, K10N, N20K, Q43H, T90E

K10N

The correct answer is Option (2) → K10N

Letter pattern:

• First letters increase by +3: H → K → N → Q → T

• Last letters decrease by −3: Q → N → K → H → E

Pattern in the numbers

The numbers follow the rule:
aₙ = 2 × aₙ₋₁ + (n − 1), where n is the term position.
where n is the term position.

a₁ = 4

a₂ = 2 × 4 + 1 = 9 (but the series shows 10, which breaks the pattern)

Continuing with the correct rule:

a₃ = 2 × 9 + 2 = 20
a₄ = 2 × 20 + 3 = 43
a₅ = 2 × 43 + 4 = 90

Thus, the second term should contain 9, not 10.

Conclusion: K10N breaks the numerical pattern while all other parts of the series follow the rule.