# What is the z-score of sample X, if n = 36, mu= 12.3, St. Dev. =21, and mu_X =18.5?

$z = 1.77$
$z = \frac{\mu - {\mu}_{x}}{S E}$
$S E = \frac{\sigma}{\sqrt{n}} = \frac{21}{\sqrt{36}} = \frac{21}{6} = 3.5$
$z = \frac{12.3 - 18.5}{3.5} = \frac{- 6.2}{3.5} = - 1.77$