Difficulty: Easy
Correct Answer: 15
Explanation:
Introduction / Context:
Counting primes in a small interval checks quick recall of basic primes and composite recognition. The range 0 to 50 is common in aptitude tests and can be handled by enumeration or a mental sieve of Eratosthenes.
Given Data / Assumptions:
Concept / Approach:
List all primes under 50 and count them carefully, ensuring none are missed or misclassified.
Step-by-Step Solution:
Verification / Alternative check:
Mental sieve: strike out even numbers > 2, multiples of 3, of 5, and of 7; the remaining list matches the primes above.
Why Other Options Are Wrong:
13, 14, and 16 miscount by omitting or including numbers incorrectly; 12 is the count under 40, not under 50.
Common Pitfalls:
Including 1 as prime or forgetting 41, 43, and 47.
Final Answer:
15
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