A number is chosen uniformly at random from {1, 2, 3, …, 50}. What is the probability that the chosen number is prime?
Aptitude
Probability
Difficulty: Easy
Choose an option
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A0.18
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B0.24
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C0.30
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D0.36
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ENone of these
Answer
Correct Answer: 0.30
Explanation
Introduction / Context:Prime-counting over a small range is straightforward by enumeration or recalling standard lists up to 50. Probability equals (number of primes up to 50)/50 for uniform selection from 1–50.
Given Data / Assumptions:
- Sample space: integers 1 through 50 (size 50).
- Primes in this range are to be counted exactly once.
Concept / Approach:
- List primes ≤ 50.
- Compute probability = count / 50.
Step-by-Step Solution:
Primes ≤ 50: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47 (15 numbers)Probability = 15 / 50 = 3 / 10 = 0.30Verification / Alternative check:Cross-check by excluding composites and 1; the remaining count remains 15, a standard fact set for small prime tables.
Why Other Options Are Wrong:
- 0.18, 0.24, 0.36 reflect incorrect prime counts.
- “None of these” is wrong because 0.30 is correct.
Common Pitfalls:
- Forgetting that 1 is not prime and that 49 (= 7^2) is composite.
Final Answer:0.30