In this number-pair odd-man-out question, select the pair of numbers that does not follow the same difference pattern.

Introduction / Context:
This question involves simple number pairs where the second number is related to the first by addition. In three of the pairs, the second number is obtained by adding the same constant to the first, while in one pair, a different increment is used. The objective is to identify the pair that does not fit the common difference pattern and mark it as the odd-man-out.

Given Data / Assumptions:

• Number pairs: 31 – 33, 33 – 35, 37 – 39 and 35 – 39.
• The first number is on the left of the dash, and the second on the right.
• We check how much is added to the first number to obtain the second number in each pair.

Concept / Approach:
The obvious thing to check here is the difference between numbers in each pair. For three of the options, the jump is constant at +2. If one pair shows a different jump, that pair must be the odd one out. Questions based on constant differences are intended to be solved quickly using mental arithmetic.

Step-by-Step Solution:
Step 1: For the pair 31 – 33, compute the difference: 33 - 31 = 2. So the rule is second = first + 2. Step 2: For the pair 33 – 35, compute the difference: 35 - 33 = 2. This pair again follows second = first + 2. Step 3: For the pair 37 – 39, compute the difference: 39 - 37 = 2. This also fits the same pattern second = first + 2. Step 4: For the pair 35 – 39, compute the difference: 39 - 35 = 4. This time the difference is +4, not +2. Step 5: Summarise. Three pairs show a constant difference of +2 between the first and second numbers. One pair shows a larger difference of +4. Step 6: Conclude that 35 – 39 is the odd pair because its difference is different from the others.

Verification / Alternative check:
We can also think of these pairs as representing consecutive odd numbers. 31 and 33 are consecutive odd numbers, as are 33 and 35, and 37 and 39. The pair 35 and 39, however, skips 37 and thus does not consist of consecutive odd numbers. So we have two equivalent ways of expressing the same idea: a difference of 2 or consecutive odd numbers. Both views select the same pair, 35 – 39, as the odd-man-out.

Why Other Options Are Wrong:
31 – 33 is not odd because it uses a difference of 2 and represents consecutive odd numbers. 33 – 35 also has a difference of 2. 37 – 39 fits the same pattern. These three pairs all show the same simple relationship and thus belong to one group. Only 35 – 39 has a difference of 4 and skips an odd number, so the other pairs cannot be considered the odd ones.

Common Pitfalls:
Some learners may confuse themselves by focusing on individual properties of the numbers, such as whether they are prime or composite. For example, 31 and 37 are prime, while 33, 35 and 39 are composite, but this does not help classify the pairs consistently. The simpler and intended pattern is the constant difference between the numbers. Always check for basic arithmetic relationships first in such questions, as they are usually enough to solve the problem quickly.

Final Answer:
The odd number pair is 35 – 39, because the difference between the two numbers is 4, whereas in all the other pairs the difference is only 2.