In the number pairs 140 → 45, 110 → 35, 100 → 30 and 80 → 25, identify the odd pair based on the final digit of the second number.

Introduction / Context:
This odd one out question uses number pairs where the second number follows a simple pattern in its last digit. Instead of a complicated arithmetic rule, the question can be solved by observing the units place of the second number in each pair. Such problems are designed to test quick visual recognition of patterns and basic number properties rather than lengthy calculations.

Given Data / Assumptions:
The four number pairs are 140 → 45, 110 → 35, 100 → 30 and 80 → 25. In each pair, the first number is mapped to a second number by an unstated rule. We focus on the second numbers 45, 35, 30 and 25. We look specifically at the units digit of these second numbers to detect a pattern.

Concept / Approach:
A simple but effective approach is to check whether the second numbers share a common units digit or a small set of allowed digits. In this question, three of the second numbers end in the digit 5, while one of them ends in 0. This difference in the units place gives a clear and fast rule for identifying the odd pair. Such pattern based reasoning is often used in aptitude tests to reward careful observation over heavy arithmetic.

Step-by-Step Solution:
Step 1: Look at the second number in the first pair, 45. Its units digit is 5. Step 2: For the second pair, 110 → 35, the second number is 35 and its units digit is also 5. Step 3: For the fourth pair, 80 → 25, the second number is 25 and its units digit is again 5. Step 4: For the third pair, 100 → 30, the second number is 30 and its units digit is 0, not 5. Step 5: Therefore, three of the second numbers end with 5, while only one ends with 0. Step 6: The pair whose second number does not end with the digit 5 is 100 → 30, so this pair is the odd one out.

Verification / Alternative check:
As a quick verification, write the second numbers vertically: 45, 35, 30 and 25, and inspect their last digits. You will immediately see 5, 5, 0 and 5, where only one number has a different units digit. No further arithmetic is necessary, and there is no alternative simple rule that separates one of the other pairs as clearly as this units digit rule does.

Why Other Options Are Wrong:
140 → 45 is not the odd pair because 45 ends with 5, matching the pattern of most second numbers. 110 → 35 is not the odd pair because 35 also ends with 5. 80 → 25 is not the odd pair because 25 again ends with 5. 100 → 30 is the odd pair because its second number ends with 0 instead of 5.

Common Pitfalls:
Many learners initially try to connect the first and second numbers by complex operations such as multiplication, division or ratios and may overlook the simple units digit pattern. Another pitfall is to treat the arrow as implying a special algebraic formula, when the exam actually tests observation rather than heavy calculation. To avoid wasting time, always check for simple properties such as parity, last digit or divisibility before exploring more complicated patterns.

Final Answer:
The pair whose second number does not end with the digit 5 and is therefore the odd one out is 100 → 30.