Answer
$P(x < 20) = 0.1587$
Work Step by Step
Want to find $P(x < 20$)
i) Convert 20 to a z-score:
z = $\frac{x - \mu}{\sigma}$
z = $\frac{20 - 21}{1}$
z = -1
ii) $P(x< 20) = P(z < -1)$ = 0.1587 ( read directly from the standard normal table)
