Question 551048
<pre>
z = {{{(xbar - mu)/(sigma/sqrt(n))}}} where {{{sigma}}} = {{{sqrt(variance)}}}

and where when n > 30, we use the approximation s &#8776; {{{sigma}}}.

So in this problem we use {{{sigma}}} &#8776; s = {{{sqrt(25)}}} = 5

z = {{{(xbar - mu)/(sigma/sqrt(n))}}} = {{{(30-15)/(5/sqrt(50))}}} = 21.21320344.

That's an extremely high z-score but that's what you would 
expect since 30 is so extremely far above the mean of 15.

Edwin<pre>