Jon Jackobsen, an over jealous graduate student, has just completed a first draft of his 700-page dissertation. Jon has typed his paper himself and interested in knowing the average number of typographical errors per page, but does not want to read the whole paper. Knowing a little bit about Business Statistics, Jon selected 40 pages at random to read and found that the average number of typos per page was 4.3 and the sample standard deviation was 1.2 typos per page.

Calculate the estimated standard error of the mean.

Construct for Jon a 90 percent confidence interval for the true average number of typos per page.