Among the numbers 5712, 6212, 3811 and 7831, identify the odd one out based on the count of odd and even digits.

Introduction / Context:
In some odd one out questions, the hidden pattern is not about divisibility or digit sums but about the parity pattern of the digits, that is, whether the digits are odd or even. Such questions are popular in reasoning sections because they can be solved quickly once you look at the correct feature. In this problem, we have four four digit numbers that at first seem unrelated, but one of them has a different combination of odd and even digits compared to the others.

Given Data / Assumptions:
The numbers given are 5712, 6212, 3811 and 7831. Each number has four digits. We classify each digit as odd or even. We compare the count of odd digits and even digits in each number.

Concept / Approach:
The concept is to look at the parity pattern across all four digits in each number. In three of the numbers, the pattern is three odd digits and one even digit. In one of the numbers, the pattern is reversed with three even digits and one odd digit. The number with the different pattern becomes the odd one out. This method does not require heavy calculations and relies only on recognizing whether each digit is odd or even, which is ideal for fast reasoning in competitive exams.

Step-by-Step Solution:
Step 1: Consider 5712. The digits are 5, 7, 1 and 2. Here 5 is odd, 7 is odd, 1 is odd and 2 is even. That means this number has three odd digits and one even digit. Step 2: Consider 3811. The digits are 3, 8, 1 and 1. Here 3 is odd, 8 is even and the two 1s are odd, so again we have three odd digits and one even digit. Step 3: Consider 7831. The digits are 7, 8, 3 and 1. Here 7, 3 and 1 are odd, while 8 is even, so this number also has three odd digits and one even digit. Step 4: Consider 6212. The digits are 6, 2, 1 and 2. In this case, 6 is even, 2 is even, the next 1 is odd and the last 2 is even. So there are three even digits and one odd digit. Step 5: Therefore, 5712, 3811 and 7831 share a common pattern of three odd digits and one even digit, while 6212 has the opposite pattern of three even digits and one odd digit. So 6212 is the odd one out.

Verification / Alternative check:
We can verify by simply recounting for each number: 5712 has odd, odd, odd, even; 3811 has odd, even, odd, odd; 7831 has odd, even, odd, odd; and 6212 has even, even, odd, even. This confirms that only 6212 does not fit the three odd one even pattern. No simple divisibility or digit sum rule separates the numbers as neatly as this parity pattern, which makes it the intended logic for exam use.

Why Other Options Are Wrong:
5712 is not the odd one out because it has three odd digits and one even digit, just like 3811 and 7831. 3811 is not the odd one out because its parity pattern matches that of 5712 and 7831. 7831 is not the odd one out because it also has three odd digits and one even digit. 6212 is different from all three because it has three even digits and one odd digit, which reverses the common pattern.

Common Pitfalls:
Many learners first try divisibility by 2, 3, 5 or 11 for such number sets and may waste time without finding a clear distinction. Another common mistake is to look only at the first or last digit and ignore the total pattern across all digits. To avoid these issues, it is useful to quickly scan for patterns in parity across all digits whenever you see multi digit odd one out questions.

Final Answer:
The number whose combination of odd and even digits is different and is therefore the odd one out is 6212.