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Question

Find the number that doesn't belong:

This question was previously asked in
SSC Stenographer 2025 Question Paper (06-Aug-2025) Shift 2
The correct answer is
111

Number Doesn't Belong: Prime Number Analysis

The question asks us to identify the number from the given list that does not share a common property with the others. The list of numbers is 103, 109, 107, and 111.

Analyzing Number Properties

To find the number that doesn't belong, let's examine the properties of each number, particularly focusing on whether they are prime or composite.

  • Prime Numbers: A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself.
  • Composite Numbers: A composite number is a natural number that has more than two positive divisors.

Checking Each Number

  • 103: Let's check for divisors. It's not divisible by 2, 3 (sum of digits = 4), 5, 7 (103 / 7 ≈ 14.7), 11 (alternating sum = 1-0+3 = 4). The square root of 103 is approximately 10.1. We only need to check primes up to 10 (2, 3, 5, 7). Since it's not divisible by any of these, 103 is a prime number.
  • 109: Checking for divisors: not divisible by 2, 3 (sum = 10), 5, 7 (109 / 7 ≈ 15.5), 11 (alternating sum = 1-0+9 = 10). The square root of 109 is approximately 10.4. We check primes up to 10 (2, 3, 5, 7). Since it's not divisible by these, 109 is a prime number.
  • 107: Checking for divisors: not divisible by 2, 3 (sum = 8), 5, 7 (107 / 7 ≈ 15.3), 11 (alternating sum = 1-0+7 = 8). The square root of 107 is approximately 10.3. We check primes up to 10 (2, 3, 5, 7). Since it's not divisible by these, 107 is a prime number.
  • 111: Checking for divisors:
    • Is it divisible by 2? No, it's an odd number.
    • Is it divisible by 3? Let's sum the digits: \(1 + 1 + 1 = 3\). Since 3 is divisible by 3, 111 is divisible by 3.
    • Calculation: \(111 \div 3 = 37\).
    Since 111 has divisors other than 1 and itself (namely 3 and 37), 111 is a composite number.

Conclusion

Comparing the numbers:

  • 103 is prime.
  • 109 is prime.
  • 107 is prime.
  • 111 is composite.

The numbers 103, 109, and 107 are all prime numbers. The number 111 is a composite number because it is divisible by 3 and 37.

Therefore, the number that doesn't belong in the list is 111.

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Similar Questions

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Important Questions from Classification

  1. Three of the following four are alike in a certain way and thus form a group. Which is the one that does NOT belong to that group?
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  2. Three of the following numbers are alike in a certain way and one is different. Pick the odd one out.

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  3. The following contains two pairs of words which are related to each other in a certain way. Three of the following four word-pairs are alike as these have the same relationship and thus form a group. Which word-pair does NOT belong to that group?
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  4. Three of the following four options are alike in a certain way and thus form a group. Which is the one that does NOT belong to that group?

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  5. The following contains two pairs of words which are related to each other in a certain way. Three of the following four word-pairs are alike as these have the same relationship and thus form a group. Which word-pair does NOT belong to that group?
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