NECO 2005 Mathematics · Question 9

Two fair dice are tossed together once. What is the probability of getting a total of at least 9 from the outcome?

Options

  1. A.19\frac{1}{9}
  2. B.16\frac{1}{6}
  3. C.518\frac{5}{18}
  4. D.13\frac{1}{3}
  5. E.1318\frac{13}{18}

Answer: C. 518\frac{5}{18}

Explanation written with AI and checked against the answer key

Analyze the question: Two fair dice are tossed, and we want P(sum ≥ 9). The total number of outcomes is 6 × 6 = 36.

Concept: Probability = (favourable outcomes) ÷ (total outcomes). Count the ways to get each total from 9 to 12.

Solution:

  • Sum = 9: (3,6), (4,5), (5,4), (6,3), which is 4 ways
  • Sum = 10: (4,6), (5,5), (6,4), which is 3 ways
  • Sum = 11: (5,6), (6,5), which is 2 ways
  • Sum = 12: (6,6), which is 1 way

Total favourable = 4 + 3 + 2 + 1 = 10

P(sum ≥ 9) = 10/36 = 5/18

Answer: Option C, 5/18.

Tip: For dice sums, the number of ways rises by 1 up to 7 and then falls by 1. The counts for sums 2 to 12 are 1, 2, 3, 4, 5, 6, 5, 4, 3, 2, 1. For "at least 9", add the last four: 4+3+2+1 = 10. Always simplify the fraction over 36.

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