Maths Olympiad Prep

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Algebra Difficulty 5.5 AIME, harder Find the answer United States

Problem:

Marisa has two identical cubical dice labeled with the numbers {1,2,3,4,5,6}\{1,2,3,4,5,6\}. However, the two dice are not fair, meaning that they can land on each face with different probability. Marisa rolls the two dice and calculates their sum. Given that the sum is 2 with probability 0.04, and 12 with probability 0.01, the maximum possible probability of the sum being 7 is pp. Compute 100p\lfloor 100 p\rfloor.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution:

Let pip_{i} be the probability that the die lands on the number ii. The problem gives that p12=0.04p_{1}^{2}=0.04, p62=0.01p_{6}^{2}=0.01, so we have
p1=0.2,p6=0.1,p2+p3+p4+p5=0.7 p_{1}=0.2, \quad p_{6}=0.1, \quad p_{2}+p_{3}+p_{4}+p_{5}=0.7
We are asked to maximize
2(p1p6+p2p5+p3p4)=2(0.02+p2p5+p3p4) 2\left(p_{1} p_{6}+p_{2} p_{5}+p_{3} p_{4}\right)=2\left(0.02+p_{2} p_{5}+p_{3} p_{4}\right)
Let x=p2+p5x=p_{2}+p_{5} and y=p3+p4y=p_{3}+p_{4}. Then by AM-GM, p2p5x24p_{2} p_{5} \leq \frac{x^{2}}{4}, p3p4y24p_{3} p_{4} \leq \frac{y^{2}}{4}. Also,
x2+y24x2+2xy+y24=(x+y)24=0.724=0.1225 \frac{x^{2}+y^{2}}{4} \leq \frac{x^{2}+2 x y+y^{2}}{4}=\frac{(x+y)^{2}}{4}=\frac{0.7^{2}}{4}=0.1225
Hence,
2(p1p6+p2p5+p3p4)2(0.02+0.1225)=0.285 2\left(p_{1} p_{6}+p_{2} p_{5}+p_{3} p_{4}\right) \leq 2(0.02+0.1225)=0.285
where equality holds if p2=p5=0.35p_{2}=p_{5}=0.35, p3=p4=0p_{3}=p_{4}=0.

Thus, we conclude that p=0.285p=0.285 and 100p=28\lfloor 100 p\rfloor=28.

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