Answer
According to the model, $20$-year-old tree has a diameter of $1.6 feet$
Work Step by Step
We have the following function:
$D(t)=\frac{5.4}{1+2.9e^{-0.01t}}$
Which represents a diameter of a $t$ years old tree.
For $20$-years-old tree ($t=20$) we have:
$D(20)=\frac{5.4}{1+2.9e^{-0.01\times20}}=\frac{5.4}{1+2.9e^{-0.2}}\approx1.60032\approx1.6$ feet
