In the letter pair series DK, FN, HQ, ?, which of the following pairs should replace the question mark to continue the pattern?

Introduction / Context:
This problem asks you to complete a letter pair series: DK, FN, HQ, ?. Such series usually involve regular steps in the alphabet for both the first letters and the second letters. Recognising these steps allows you to predict the next pair quickly.

Given Data / Assumptions:

• Given pairs: DK, FN, HQ, ?
• First letters: D, F, H, ?
• Second letters: K, N, Q, ?
• Alphabet positions A 1 to Z 26.

Concept / Approach:
We treat the first letters of each pair as one numeric sequence and the second letters as another. In many exam questions, both sequences have a constant difference, though the differences may be different for the first and second letters. After converting to numbers and finding the step size, we extend each sequence by one step to obtain both letters of the missing pair.

Step-by-Step Solution:
Step 1: Convert first letters D, F, H to numeric positions: D 4, F 6, H 8. Step 2: The differences are +2 in each step (4 to 6, 6 to 8). Step 3: Continue the pattern by adding 2 to 8. This gives 10, which corresponds to J. So the first letter of the missing pair is J. Step 4: Now convert second letters K, N, Q to numbers: K 11, N 14, Q 17. Step 5: The differences here are +3 each time (11 to 14, 14 to 17). Step 6: Extend this sequence by adding 3 to 17, giving 20, which corresponds to T. So the second letter of the missing pair is T. Step 7: Therefore, the required pair is JT.

Verification / Alternative check:
We can check all pairs together: DK (4,11), FN (6,14), HQ (8,17), JT (10,20). In the first column, we have 4, 6, 8, 10, a simple plus 2 progression. In the second column, we have 11, 14, 17, 20, a simple plus 3 progression. No other suggested pair creates this consistent pattern across both columns.

Why Other Options Are Wrong:
KS would give first letter K 11, which does not follow the plus 2 sequence 4, 6, 8, 10. KT has second letter T correct but first letter K incorrect. JS gets the first letter right (J 10) but second letter S 19 breaks the plus 3 sequence 11, 14, 17, 20. JU similarly gives a second letter U 21, which does not match the required 20.

Common Pitfalls:
One common mistake is to look only at the second letters or only at the first letters, and then choose a pair that satisfies only one of the patterns. Another pitfall is guessing based on approximate positions rather than computing exact numeric differences. Always convert letters, compute the difference for each sequence, and verify that a candidate pair preserves both patterns.

Final Answer:
The pair that continues the series correctly is JT.