In the following alphabet coding question, four letter pairs are given in the form AB – C: BN – P, GI – R, LM – Y and TA – U. In three of these, the single letter represents the sum of the positions of the two letters on the left, while one pair breaks this rule. Select the odd letter pair from the given alternatives.

Introduction / Context:
This question belongs to the alphabet coding and analogy section. You are given four patterns of the form two letters followed by a single letter: BN – P, GI – R, LM – Y and TA – U. In such questions, the single letter is usually derived from some arithmetic operation on the positions of the two letters in the alphabet. Here, three pairs follow the rule that the position of the single letter equals the sum of the positions of the two letters on the left. One pair does not follow this rule and must be selected as the odd one out.

Given Data / Assumptions:

• Pairs: BN – P, GI – R, LM – Y and TA – U.
• Alphabet positions: A = 1, B = 2, C = 3, ..., Z = 26.
• For each pair AB – C, we compute position(A) + position(B) and compare with position(C).
• Three pairs satisfy position(C) = position(A) + position(B).
• One pair does not satisfy this rule.

Concept / Approach:
The concept is to convert letters to numbers and perform addition. For example, in BN – P, we check if B (2) plus N (14) gives P (16). If this holds, then the pattern is consistent. We repeat this process for all four pairs. The pair where the sum of the first two letters does not equal the position of the third letter is the odd one out. This simple arithmetic based coding is very common in verbal reasoning tests.

Step-by-Step Solution:
Step 1: Analyse BN – P.B = 2 and N = 14. Sum = 2 + 14 = 16. P is the 16th letter of the alphabet. So BN – P satisfies the rule.Step 2: Analyse LM – Y.L = 12 and M = 13. Sum = 12 + 13 = 25. Y is the 25th letter of the alphabet. So LM – Y also satisfies the rule.Step 3: Analyse TA – U.T = 20 and A = 1. Sum = 20 + 1 = 21. U is the 21st letter of the alphabet. So TA – U again follows the rule.Step 4: Analyse GI – R.G = 7 and I = 9. Sum = 7 + 9 = 16. However, 16th letter is P, not R. R is the 18th letter, so GI – R does not satisfy the rule.Step 5: Identify the odd pair.BN – P, LM – Y and TA – U all match the sum of positions rule. GI – R is the only pair that does not, so it is the odd one out.

Verification / Alternative check:
You can verify quickly by writing down the alphabet with positions and checking each pair mentally. For GI – R, once you notice that G + I gives 16 (P) but the pair shows R, which is position 18, it is clear that the coding is inconsistent. Ensuring that all three other pairs work perfectly with the sum rule further confirms that GI – R is the unique exception.

Why Other Options Are Wrong:
BN – P: 2 + 14 = 16 and P is the 16th letter, so it follows the rule.
LM – Y: 12 + 13 = 25 and Y is the 25th letter, so it is also consistent.
TA – U: 20 + 1 = 21 and U is the 21st letter, so this pair belongs to the pattern.

Common Pitfalls:
A common error is to check only one or two pairs and assume all follow the same pattern, missing the incorrect one. Another pitfall is miscounting letter positions, especially in the second half of the alphabet. To avoid mistakes, it is useful to remember positions of a few key letters, such as J = 10, M = 13, P = 16, T = 20 and Z = 26, and use them as anchors when calculating others.

Final Answer:
The odd letter pair is GI – R, because it does not follow the rule that the position of the third letter equals the sum of the positions of the first two letters.