Answer:
[3.75 years, 5.45 years]
Step-by-step explanation:
The 90% confidence interval is given by the interval
where
<em> is the sample mean </em>
<em>s is the sample standard deviation </em>
<em>n is the sample size
</em>
is the 0.05 (5%) both-sided (**) critical value for <em>the Student's t-distribution with 19 degrees of freedom (sample size -1), which is an approximation to the Normal distribution for small samples (n≤ 30).
</em>
Either by using a table or the computer, we find
and our 90% confidence interval is
(**) 5% of the area to the left of and 5% to right of for a total of 90% inside the interval