Maths Olympiad Prep

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Algebra Difficulty 5.2 AIME, harder Prove it United States

Problem:

Two dice are loaded so that the numbers 11 through 66 come up with various (possibly different) probabilities on each die. Is it possible that, when both dice are rolled, each of the possible totals 22 through 1212 has an equal probability of occurring?

Solution

Solution:

The answer is no. Suppose that each of the totals from 22 to 1212 has an equal probability, which must be 1/111/11 since the sum of all probabilities is 11. Let aa and bb be the probabilities of a 11 and a 66, respectively, on the first die, and let cc and dd be the corresponding probabilities on the second die.

Since 1/111/11 is the probability of rolling a total of 22, ac=1/11a c = 1/11 so c=1/(11a)c = 1/(11a); since 1/111/11 is the probability of rolling 1212, bd=1/11b d = 1/11 so d=1/(11b)d = 1/(11b). Since the probability of rolling a 77 through the combination 1+61+6 or 6+16+1 is at most 1/111/11,
111ad+bc111a11b+b11a1ab+ba \begin{aligned} \frac{1}{11} &\geq a d + b c \\ \frac{1}{11} &\geq \frac{a}{11b} + \frac{b}{11a} \\ 1 &\geq \frac{a}{b} + \frac{b}{a} \end{aligned}
Since a/ba/b and b/ab/a are reciprocals, one of them is at least 11, so this inequality cannot hold.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.