AlgebraDifficulty 5.5AIME, harderFind the answerUnited States
Problem:
Marisa has two identical cubical dice labeled with the numbers {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 p. Compute ⌊100p⌋.
A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.
Solution
Solution:
Let pi be the probability that the die lands on the number i. The problem gives that p12=0.04, p62=0.01, so we have p1=0.2,p6=0.1,p2+p3+p4+p5=0.7 We are asked to maximize 2(p1p6+p2p5+p3p4)=2(0.02+p2p5+p3p4) Let x=p2+p5 and y=p3+p4. Then by AM-GM, p2p5≤4x2, p3p4≤4y2. Also, 4x2+y2≤4x2+2xy+y2=4(x+y)2=40.72=0.1225 Hence, 2(p1p6+p2p5+p3p4)≤2(0.02+0.1225)=0.285 where equality holds if p2=p5=0.35, p3=p4=0.
Thus, we conclude that p=0.285 and ⌊100p⌋=28.
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Source: MathNet,
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