Q266 / 269 · NDA 2025 (Phase 1) · Mathematics

If two fair dice are tossed, then what is the probability that the sum of the numbers on the faces of the dice is strictly greater than 7?

  • A
    $\dfrac{1}{3}$
  • Correct answer
    B
    $\dfrac{5}{12}$
  • C
    $\dfrac{7}{12}$
  • D
    $\dfrac{3}{4}$

Solution

Total number of possible outcomes when two fair dice are tossed is $6 \times 6 = 36$.

Let $S$ be the sum of the numbers on the faces of the dice. By symmetry, the probability of getting a sum strictly greater than $7$ is equal to the probability of getting a sum strictly less than $7$.

$P(S > 7) + P(S < 7) + P(S = 7) = 1$

The outcomes for $S = 7$ are $(1,6), (2,5), (3,4), (4,3), (5,2), (6,1)$, which are $6$ in number. Thus, $P(S = 7) = \dfrac{6}{36} = \dfrac{1}{6}$.

$2 P(S > 7) + \dfrac{1}{6} = 1$

$2 P(S > 7) = 1 - \dfrac{1}{6} = \dfrac{5}{6}$

$P(S > 7) = \dfrac{5}{12}$

Answer: $\dfrac{5}{12}$

Hence, the correct answer is option: 'B'.

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