Single correct option
Two fair dice are thrown. What is the probability that the sum of the numbers shown is a prime number?
- A. 1/3
- B. 5/12
- C. 7/18
- D. 1/2
Show answer and solution
Answer: (B) 5/12
The possible prime sums are 2, 3, 5, 7 and 11.
Ways: sum 2 → 1, sum 3 → 2, sum 5 → 4, sum 7 → 6, sum 11 → 2. Total = 15.
Probability = 15/36 = 5/12.
Tip: the number of ways to roll sum s with two dice is s − 1 for s ≤ 7 and 13 − s for s ≥ 7.
Ways: sum 2 → 1, sum 3 → 2, sum 5 → 4, sum 7 → 6, sum 11 → 2. Total = 15.
Probability = 15/36 = 5/12.
Tip: the number of ways to roll sum s with two dice is s − 1 for s ≤ 7 and 13 − s for s ≥ 7.
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