AIME 2014 I · 第 2 题

AIME 2014 I — Problem 2

专题: Contest Math
难度: L4
来源: AIME

题目详情

Problem 2

An urn contains $4$ green balls and $6$ blue balls. A second urn contains $16$ green balls and $N$ blue balls. A single ball is drawn at random from each urn. The probability that both balls are of the same color is $0.58$ . Find $N$ .

解析

Solution

First, we find the probability both are green, then the probability both are blue, and add the two probabilities. The sum should be equal to $0.58$ .

The probability both are green is $\frac{4}{10}\cdot\frac{16}{16+N}$ , and the probability both are blue is $\frac{6}{10}\cdot\frac{N}{16+N}$ , so

$\frac{4}{10}\cdot\frac{16}{16+N}+\frac{6}{10}\cdot\frac{N}{16+N}=\frac{29}{50}$ Solving this equation,

$20\left(\frac{16}{16+N}\right)+30\left(\frac{N}{16+N}\right)=29$ Multiplying both sides by $16+N$ , we get

\begin{aligned} 20\cdot16+30\cdot N&=29(16+N)\\ 320+30N&=464+29N\\ N&=\boxed{144} \end{aligned}