2024 AMC 12B 第 25 题

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25.

Pablo 要用红色或蓝色颜料、条纹或圆点图案装饰 66 个相同的白球。他会对每个球的颜色和图案各掷一次公平硬币,共 1212 次决策。颜料干后,他把 66 个球放入一个盒子。Frida 从盒子中随机取一个球,并记录其颜色和图案。事件“Frida 取到的球是红色”和“Frida 取到的球是条纹”是否独立,取决于 Pablo 的掷硬币结果。这两个事件独立的概率可写为 mn\dfrac{m}{n},其中 mmnn 是互质正整数。求 mm?(回忆:若两个事件 AABB 满足 P(A and B)=P(A)P(B)P(A \text{ and } B) = P(A)\cdot P(B),则它们独立。)

Pablo will decorate each of 66 identical white balls with either a striped or a dotted pattern, using either red or blue paint. He will decide on the color and pattern for each ball by flipping a fair coin for each of the 1212 decisions he must make. After the paint dries, he will place the 66 balls in an urn. Frida will randomly select one ball from the urn and note its color and pattern. The events "the ball Frida selects is red" and "the ball Frida selects is striped" may or may not be independent, depending on the outcome of Pablo's coin flips. The probability that these two events are independent can be written as mn,\dfrac{m}{n}, where mm and nn are relatively prime positive integers. What is m?m? (Recall that two events AA and BB are independent if P(A and B)=P(A)P(B).P(A \text{ and } B) = P(A)\cdot P(B).)

243243

245245

247247

249249

251251

答案:A
知识点:独立事件分类讨论
难度评级:2510
解答:

每个球独立地属于四种等可能类型之一:红色条纹、红色圆点、蓝色条纹、蓝色圆点。设 66 个球中有 kk 个红色条纹球,红球总数为 RR,条纹球总数为 SS。对 Frida 的均匀随机选择,P(red)=R6,P(\text{red}) = \tfrac{R}{6}, P(striped)=S6,P(\text{striped}) = \tfrac{S}{6},P(red and striped)=k6.P(\text{red and striped}) = \tfrac{k}{6}. 独立性意味着 k6=R6S6,\dfrac{k}{6} = \dfrac{R}{6}\cdot\dfrac{S}{6},6k=RS.6k = RS.

先计数满足 R{0,6}R\in\{0,6\}S{0,6}.S\in\{0,6\}. 的分配。四个条件各给另一种属性留下 26=642^6=64 种选择,而它们两两交集中的四个分配被重复计算。因此并集贡献 4644=252.4\cdot64-4=252.

1R,S5,1\le R,S\le5, 时,条件 6RS6\mid RS 只留下 (R,S)=(2,3),(3,2)(R,S)=(2,3),(3,2)(3,4),(4,3).(3,4),(4,3). 每种情形都有 k=RS/6,k=RS/6,四种类型的数量是 (1,1,2,2).(1,1,2,2). 的一个排列。因此每个数对贡献 6!1!1!2!2!=180\dfrac{6!}{1!1!2!2!}=180 种分配。有利分配数为 252+4180=972.252+4\cdot180=972.46=40964^6=4096 个等可能分配中,概率为 9724096=2431024,\dfrac{972}{4096}=\dfrac{243}{1024},所以 m=243.m=243.

因此,正确答案是 A

Each ball is independently one of four equally likely types: red-striped, red-dotted, blue-striped, blue-dotted. Suppose among the 66 balls there are kk red-striped, with RR red and SS striped in total. For Frida's uniform pick, P(red)=R6,P(\text{red}) = \tfrac{R}{6}, P(striped)=S6,P(\text{striped}) = \tfrac{S}{6}, and P(red and striped)=k6.P(\text{red and striped}) = \tfrac{k}{6}. Independence means k6=R6S6,\dfrac{k}{6} = \dfrac{R}{6}\cdot\dfrac{S}{6}, i.e. 6k=RS.6k = RS.

First count assignments with R{0,6}R\in\{0,6\} or S{0,6}.S\in\{0,6\}. Each of these four conditions leaves 26=642^6=64 choices for the other attribute, and the four assignments at their pairwise intersections have been counted twice. Their union therefore contributes 4644=252.4\cdot64-4=252.

For 1R,S5,1\le R,S\le5, the condition 6RS6\mid RS leaves only (R,S)=(2,3),(3,2)(R,S)=(2,3),(3,2) or (3,4),(4,3).(3,4),(4,3). In each case k=RS/6,k=RS/6, and the four type counts are a permutation of (1,1,2,2).(1,1,2,2). Thus each pair contributes 6!1!1!2!2!=180\dfrac{6!}{1!1!2!2!}=180 assignments. The favorable count is therefore 252+4180=972.252+4\cdot180=972. Out of 46=40964^6=4096 equally likely assignments, the probability is 9724096=2431024,\dfrac{972}{4096}=\dfrac{243}{1024}, so m=243.m=243.

Thus, the correct answer is A.

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