Maths Olympiad Prep

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Combinatorics Difficulty 4.8 AIME Find the answer

32. On a twelve-sided die, the numbers 1,2,3,41,2,3,4 are each marked on two faces, and the numbers 5,6,7,8 are each marked on one face. Observing, it is found that the probability of each of the 12 faces appearing is the same. The probability that the sum of the results of two rolls of this twelve-sided die is 6 is:

Pick one

Solution

32. (A).
6=5+1=4+2=3+3=2+4=1+5 6=5+1=4+2=3+3=2+4=1+5 \text {. }

Due to the probability of the first roll showing 5,4,3,2,15,4,3,2,1 being 112,16,161616\frac{1}{12}, \frac{1}{6}, \frac{1}{6} \cdot \frac{1}{6} \cdot \frac{1}{6}; the probability of the second roll showing 1,2,3,4,51,2,3,4,5 being 16,16,16,16,112\frac{1}{6}, \frac{1}{6}, \frac{1}{6}, \frac{1}{6}, \frac{1}{12}, the required probability is
112×16+16×16×3+16×112=19. \frac{1}{12} \times \frac{1}{6}+\frac{1}{6} \times \frac{1}{6} \times 3+\frac{1}{6} \times \frac{1}{12}=\frac{1}{9} .

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.