Which is different from the rest?

Appeared in: HPSSC Staff Nurse - 2021

Explanation

The number 21 is a composite number, meaning it has factors other than 1 and itself.
Specifically, 21 is divisible by 3 and 7 (3 × 7 = 21).
This property makes it different from the other options provided, which are all prime numbers.

Option B: 23 is a prime number, divisible only by 1 and 23. It fits the pattern shared by two other options.
Option C: 29 is a prime number, divisible only by 1 and 29. It fits the pattern shared by two other options.
Option D: 31 is a prime number, divisible only by 1 and 31. It fits the pattern shared by two other options.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Classification of numbers: Prime vs. Composite as background academic context rather than a clinical decision trigger.
This type of question tests basic numeracy and logical reasoning, which are foundational skills for nurses.
Nurses use numeracy daily for critical tasks like calculating medication dosages, titrating IV drips, and interpreting lab results, where a simple math error can have serious patient safety consequences.
What if the options were 9, 15, 21, and 25? In that case, all are composite numbers. The pattern might be multiples of 3 (9, 15, 21) vs. a perfect square that is not a multiple of 3 (25), making 25 the odd one out.

How to Approach the Question

First, analyze the question type. This is an 'odd one out' or classification question, requiring you to find the item that doesn't fit the pattern.
Examine each number individually to determine its mathematical properties.
Consider properties like: even/odd, prime/composite, multiples of a certain number, or perfect squares.
Identify a property that is common to three of the options but absent in the fourth.
In this case, you'll notice that 23, 29, and 31 are all prime numbers (only divisible by 1 and themselves).
The number 21 is a composite number (3 x 7), which makes it the correct answer as it is different from the rest.

Concept Tested & Keywords

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Practise the full HPSSC Staff Nurse - 2021

Attempt every question from this paper in a timed mock, then review the full solution for each one.