Question

The speeds of vehicles traveling on a highway are normally distributed with a population standard deviation of 7 miles per hour and an unknown population mean. If a random sample of 20 vehicles is taken and results in a sample mean of 60 miles per hour, find a 98% confidence interval for the population mean.

Answer 1

Answer:  $(56.03\ ,63.97)$

Step-by-step explanation:

Given : The speeds of vehicles traveling on a highway are normally distributed .

Standard deviation: $\sigma: 7$

Since , sample size : $n=20$ , which is small sample (n<30) so the test applied here is a t-test.

Degree of freedom= $n-1=20-1=19$

Sample mean : $\overline{x}=60$

Level of significance : $1-\alpha:0.98$

Then , significance level : $\alpha: 1-0.98=0.02$

Using t-distribution table , Critical value : $\text{t-score}=t_{n-1, \alpha/2}= 2.539483$

The confidence interval for population mean is given by :-

$\overline{x}\pm t_{\alpha/2}\dfrac{\sigma}{\sqrt{n}}$

$60\pm( 2.539483)\dfrac{7}{\sqrt{20}}\approx60\pm3.97=(56.03\ ,63.97)$

Hence, a 98% confidence interval for the population mean=$(56.03\ ,63.97)$