incorrect explanation of variance increase for exercise 2.27

[goretkin]

The exercise here: https://github.com/OpenIntroStat/openintro-statistics/blob/master/ch_summarizing_data/TeX/review_exercises.tex#L5

has a solution here:

openintro-statistics/extraTeX/eoceSolutions/eoceSolutions.tex

Line 381 in 70b58fc

(c)~The new score is more than 1 standard deviation away from the previous mean, so

But the justification is incorrect.

The update equation for the unbiased sample variance, supposing n-1 old measurements, and given the mean and sample variance on n-1 measurements, and a new nth measurement is:

(you can see this is identical to the second equation at https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Welford's_online_algorithm)

So the critical value of x_n so that the sample variance stays the same is

therefore, the there is a factor of 25/24 missing in the justification. I would submit a pull request, but I am not sure if the textbook covers these equations.