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Difference between revisions of "2005 Alabama ARML TST Problems/Problem 1"

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Revision as of 18:15, 17 November 2006

Problem

Two six-sided dice are constructed such that each face is equally likely to show up when rolled. The numbers on the faces of one of the dice are 1, 3, 4, 5, 6, and 8. The numbers on the faces of the other die are 1, 2, 2, 3, 3, and 4. Find the probability of rolling a sum of 9 with these two dice.

Solution

We use generating functions to represent the sum of the two dice rolls:

$(x+x^3+x^4+x^5+x^6+x^8)(x+2x^2+2x^3+x^4)=$
$x^2(1+x^2+x^3+x^4+x^5+x^7)(1+x+x^2)(1+x)$

The coefficient of $x^9$, that is, the number of ways of rolling a sum of 9, is thus $(1+2+1)=4$, out of a total of $6^2$ possible two-roll combinations, for a probability of $\frac 19$.

See also

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