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counting and probability senior
Problem
When rolling a certain unfair six-sided die with faces numbered 1, 2, 3, 4, 5, and 6, the probability of obtaining face is greater than , the probability of obtaining the face opposite face is less than , the probability of obtaining each of the other faces is , and the sum of the numbers on each pair of opposite faces is 7. When two such dice are rolled, the probability of obtaining a sum of 7 is . Given that the probability of obtaining face is , where and are relatively prime positive integers, find .
Solution
Let denote the probability of obtaining on the first die and on the second. Then the probability of obtaining a sum of 7 is Let the probability of obtaining face be . Then the probability of obtaining the face opposite face is . Therefore Then , and so . The probability of obtaining face is therefore , and .
Final answer
29