Skip to content
CalcGospel 國際數學圖譜
返回

CIE 9709 2025 June Paper 52 Q2

A Level / CIE / S1

CIE 9709 2025 June Paper 52 Paper · Question 2

题目

Problem

In Millford, 70%70\% of the residents own a bicycle. A random sample of 160 residents is selected.

Use a suitable approximation to find the probability that more than 120 of these residents own a bicycle.

(5)
题目中文翻译

在 Millford,70%70\% 的居民拥有自行车。随机抽取 160 名居民作为样本。

使用适当的近似方法,求这些居民中超过 120 人拥有自行车的概率。

(5)
原卷通用作答要求

Answer all questions. You should use a calculator where appropriate.

You must show all necessary working clearly; no marks will be given for unsupported answers from a calculator.

Give non-exact numerical answers correct to 3 significant figures, or 1 decimal place for angles in degrees, unless a different level of accuracy is specified in the question.

请回答所有题目。适当时应使用计算器。必须清楚展示所有必要的计算过程;仅给出计算器答案而没有过程,不予评分。除非题目另有精度要求,非精确数值答案保留 3 位有效数字,以度为单位的角度保留 1 位小数。

解答

解法一

思路

展开

人数服从二项分布,可用正态近似。超过120即至少121,连续性修正后的下界是120.5。

答题过程

展开

Let X∼B(160,0.7)X\sim\mathrm{B}(160,0.7) be the number owning a bicycle. Since np=112np=112 and n(1−p)=48n(1-p)=48 are both greater than 5, use Y∼N(112,33.6)Y\sim\mathrm{N}(112,33.6), where 33.6=160(0.7)(0.3)33.6=160(0.7)(0.3).

With a continuity correction,

P(X>120)= P(X≥121)≈ P(Y>120.5)= 1−Φ(120.5−11233.6)= 1−Φ(1.466389…)= 0.071271…≈ 0.0713.\begin{align*} \mathrm{P}(X>120)=&\,\mathrm{P}(X\geq121)\\[4mm] \approx&\,\mathrm{P}(Y>120.5)\\[4mm] =&\,1-\Phi\biggl(\frac{120.5-112}{\sqrt{33.6}}\biggr)\\[4mm] =&\,1-\Phi(1.466389\ldots)\\[4mm] =&\,0.071271\ldots\\[4mm] \approx&\,\boxed{0.0713}. \end{align*}