Answer
The 17th percentile for incubation times is 20 days.
Work Step by Step
$\mu$ =21, $\sigma$ = 1
i ) z score corresponding to the 70th percentile (area of 0.70) = -0.95
ii) Sub $\sigma = 1 , \mu= 21, z = -0.95$ into z = $\frac{x - \mu}{\sigma}$ and solve for x:
z = $\frac{x - \mu}{\sigma}$
$x$ = $\mu$ + $z\sigma$
$x$ = $21 + (-0.95 \times 1)$
$x$ = $20.05$ days
$x$ $\approx$ 20 days
