In the letter pairs QJ, SH, LN and UF, find the odd pair based on the sum of alphabetical positions.

Introduction / Context:
Odd letter pair questions are very common in verbal reasoning sections of aptitude tests. Each pair of letters usually follows a particular pattern based on their positions in the English alphabet. The job of the candidate is to decode the pattern and identify which option does not follow it. In this question, we have four letter pairs and we need to spot the pair that breaks a consistent numerical relationship.

Given Data / Assumptions:
The given letter pairs are QJ, SH, LN and UF. We use the standard English alphabet with positions A = 1, B = 2, ..., Z = 26. We assume that the pattern is based on numerical operations like sum or difference on these positions.

Concept / Approach:
A natural way to analyze letter pair questions is to convert each letter to its alphabetical position and then test simple operations like sum, difference, or fixed gaps. Here, when we convert each pair and add the positions of the two letters, we find that three pairs yield the same total while one pair yields a different total. The pair with the different sum is the odd one out. This fixed sum property is a classic pattern in competitive exam questions.

Step-by-Step Solution:
Step 1: Convert QJ to positions. Q is the 17th letter and J is the 10th letter. Sum = 17 + 10 = 27. Step 2: Convert SH to positions. S is 19 and H is 8. Sum = 19 + 8 = 27. Step 3: Convert LN to positions. L is 12 and N is 14. Sum = 12 + 14 = 26. Step 4: Convert UF to positions. U is 21 and F is 6. Sum = 21 + 6 = 27. Step 5: We observe that QJ, SH and UF all have a sum of 27, while LN has a sum of 26. Step 6: Therefore LN does not follow the common pattern and is the odd letter pair.

Verification / Alternative check:
Another quick check is to note directly that in three of the options the sum is 27, a convenient number to remember because it is 3^3. Once we see the three equal sums, any different total is immediately suspicious and likely to be the odd one. No other simple pattern such as constant difference or mirror positions fits all four options as neatly, so the sum rule is clearly the intended logic.

Why Other Options Are Wrong:
QJ is not odd because 17 + 10 = 27, which matches the fixed sum pattern. SH is not odd because 19 + 8 = 27, again fitting the same pattern. UF is not odd because 21 + 6 = 27, so it also belongs to the same group. Only LN breaks the pattern with a sum of 26 and therefore must be the odd one out.

Common Pitfalls:
A frequent error is to focus only on the difference between letters instead of checking their sum. That may lead to confusion because the differences are not as revealing here. Another pitfall is guessing based on intuition about consonant combinations rather than doing the simple numerical conversion. To avoid such mistakes, it is good practice to routinely convert letters to numbers and systematically test both sums and differences for letter pair questions.

Final Answer:
The letter pair that breaks the constant sum pattern and is therefore the odd one out is LN.