A fair die is rolled. The probability of getting a PRIME number is
A$1/3$
B$1/2$
C$2/3$
D$5/6$
Answer & Solution
Correct answer: B. $1/2$
1. Sample space: $\{1, 2, 3, 4, 5, 6\}$.
2. Primes among them: $2, 3, 5$. (1 is NOT prime by convention.)
3. Event 'prime': $E = \{2, 3, 5\}$, $|E| = 3$.
4. $P(\text{prime}) = 3/6 = 1/2$.
5. Note: this matches the probability of odd (B option of an earlier question). They have a different event SET but same cardinality — coincidence.
6. Option A would correspond to $|E| = 2$ (missing one prime). Option C is for $|E| = 4$. Option D is for excluding only one outcome.
_Source: NCERT Class 11 Mathematics, Ch 14, Example 1 (Prime event on die), p. 3._
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