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NDA 2025 Maths · Mathematics · Probability · easy

A fair die is thrown once. The probability of getting a prime number or a multiple of 3 is

Single correct option

A fair die is thrown once. The probability of getting a prime number or a multiple of 33 is
  1. A. 12\dfrac12
  2. B. 56\dfrac56
  3. C. 13\dfrac13
  4. D. 23\dfrac23
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Answer: (D) 23\dfrac23

Primes are {2,3,5}\{2,3,5\} and multiples of 33 are {3,6}\{3,6\}. Their union is {2,3,5,6}\{2,3,5,6\}, with 44 outcomes, so P=46=23P=\frac46=\frac23. Adding the separate probabilities 36+26=56\frac36+\frac26=\frac56 double-counts 33, a typical trap.
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Original question written in the NDA 2025 Maths pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake