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A game uses one ordinary die with faces 1 to 6 together with a special die whose six faces read 1, 1, 2, 2, 3 and 3. Both dice are rolled once. What is the probability that the number on the ordinary die is strictly greater than the number on the special die?

Competition practiceProbability & Permutation
  • A1/2
  • B2/3
  • C5/6
  • D1/6

Step-by-Step Solution

The special die shows 1, 2 or 3, each with probability 2/6 = 1/3. Take the cases in turn: against a 1 the ordinary die wins with probability 5/6; against a 2, with probability 4/6; against a 3, with probability 3/6. So the answer is (1/3) × (5/6 + 4/6 + 3/6) = (1/3) × 12/6 = 12/18 = 2/3. Counting instead over the 18 equally likely (special, ordinary) pairs gives 12 winning pairs. Trap 1/2 assumes the two dice must be evenly matched by symmetry, but the special die can never beat 3 while the ordinary one reaches 6. Trap 5/6 = 15/18 answers 'at least as large', quietly including the 3 tied pairs. Trap 1/6 = 3/18 is the probability of a tie itself.
Answer: B) 2/3

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