I have a doubt over the way my teacher solved this exercice:

Let X be a Normal random variable N (μ, σ2= 169) with an unknown parameter μ. For testing the

hypothesis H0: μ = 28 versus H1: μ = 31 a random sample of size n = 225 is observed.

a) Consider the following rejection region: ( ){ }1107,29:...,, 2251 ≥≡ xxxR . Evaluate α (the

probability of the type I error) and β (the probability of the type II error) corresponding to the given

test.

b) Calculate the p-value corresponding to the following sample:

(32, 29, 28, 34, 27)

For the point b) my teacher wrote P[Z>=(30-28)/(13/15)]

I do not understand why she used 15 when the new sample is 5 ?

Thank you.