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MathNet 2026 — Part 10 Question 75

Math Olympiad Vault · MathNet 2026 — Part 10

Question 75

In a bag are $n$ fair, six-sided dice whose faces are colored white and red in such a way that the total numbers of white and red sides are equal. Let $p$ be the probability that the same color comes up twice when taking one die randomly out of the bag and throwing it twice. Let $q$ be the probability that the same color comes up twice when taking two dice randomly out of the bag and throwing them at the same time. Prove that
$$
p+(n-1) q=\frac{n}{2}
$$