In this number based odd one out question, you must select the unique number from 2378, 9122, 1517 and 2345 by analyzing the pattern in the sum of their digits.

Introduction / Context:
This is a numerical odd one out question where the objective is to choose the number that does not satisfy a certain pattern in its digit sum. Problems like this test a candidate ability to observe properties of numbers, such as sum of digits, parity, or divisibility. Here, the standard trick is to compute the sum of the digits of each number and check for a consistent pattern among most of them.

Given Data / Assumptions:

• The given four numbers are 2378, 9122, 1517 and 2345.
• We work with the base 10 representation of each number and inspect individual digits.
• We assume that the intended pattern is based on the sum of digits.

Concept / Approach:
Digit sum is a very common tool to identify patterns in odd one out questions, especially when numbers have several digits but nothing obviously special like being prime or perfect squares. The plan is to add the digits of each number and then compare the results. If three numbers share the same digit sum and one number produces a different digit sum, then that number is the odd one out.

Step-by-Step Solution:
Step 1: For 2378, the digits are 2, 3, 7 and 8. Sum = 2 + 3 + 7 + 8 = 20.Step 2: For 9122, the digits are 9, 1, 2 and 2. Sum = 9 + 1 + 2 + 2 = 14.Step 3: For 1517, the digits are 1, 5, 1 and 7. Sum = 1 + 5 + 1 + 7 = 14.Step 4: For 2345, the digits are 2, 3, 4 and 5. Sum = 2 + 3 + 4 + 5 = 14.Step 5: We observe that three numbers, 9122, 1517 and 2345, all have a digit sum of 14, whereas 2378 has a digit sum of 20. Thus 2378 does not fit the common digit sum pattern.
Verification / Alternative check:
You can re add the digits quickly to verify there was no arithmetic mistake. Another check is to use divisibility by 9 or 3, since the digit sum mod 9 gives the remainder on division by 9. All of 9122, 1517 and 2345 share the same digit sum, so they also share the same remainder when divided by 9, while 2378 differs. This reinforces that 2378 stands apart according to digit sum criteria.

Why Other Options Are Wrong:
9122 is not the odd one out because its digit sum is 14, the same as 1517 and 2345.
1517 is also not unique, as it shares a digit sum of 14 with 9122 and 2345.
2345 again has a digit sum of 14, so it conforms to the dominant pattern.

Common Pitfalls:
A common mistake is to look for more complicated properties such as prime status or factors, which can waste time. Another error is miscalculating digit sums, especially when doing mental arithmetic quickly. Candidates should add digits carefully and double check quickly. In many aptitude exams, simple digit sum patterns like this are frequent, so practicing them helps build speed and accuracy.

Final Answer:
The only number whose digits sum to 20 instead of 14 is 2378, so 2378 is the odd one out.