One method of calculating the approximate age, in years, of a tree of a particular species is to multiply the diameter of the tree, in inches, by a constant called the growth factor for that species. The table below gives the growth factors for eight species of trees.
| Species of tree | Growth factor |
|---|---|
| Red maple | 4.5 |
| River birch | 3.5 |
| Cottonwood | 2.0 |
| Black walnut | 4.5 |
| White birch | 5.0 |
| American elm | 4.0 |
| Pin oak | 3.0 |
| Shagbark hickory | 7.5 |
A young Red maple tree has a diameter of 8 inches. Based on the information provided, what is its approximate age in years?
12.5
18.0
36.0
40.5
