Answer
at about age 56.
Work Step by Step
We solve for x when P(x)=$70\%=0.70$
$0.7=\displaystyle \frac{0.9}{1+271e^{-0.122t}}\qquad.../\times\frac{10(1+271e^{-0.122t})}{7}$
$1+271e^{-0122\mathrm{r}}=\displaystyle \frac{9}{7}\qquad.../-1\qquad(-\frac{7}{7})$
$271 \mathrm{e}^{-0.122t} =\displaystyle \frac{2}{7}\ \qquad.../\div 271$
$e^{-0.122t}=\displaystyle \frac{2}{1897}\qquad .../$apply ln( ) to both sides
$-0.122t=\displaystyle \ln\frac{2}{1897}$
$ t=\displaystyle \frac{\ln\frac{2}{1897}}{-0.122}\approx$56.1875556506
The probability of some coronary heart disease is 50\%
at about age 56.