The number of trees per acre N of a certain species is approximated by the model
N=68(10^-0.04x), 5 <_ x <_ 40 (<_ = less than or = to)
Where x is the average diameter of the trees (in inches) three feet above the ground. Use the model to approximate the average diameter of the trees in a test plot when N=21.
To solve this problem, you want to know x when N=21. Thus your first step is to ...
An exponential equation is used to model the average diameter of a group of trees.