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 certain tree species, not listed in the table, has a constant growth factor of . If a tree of this species currently has a diameter of inches and is years old, which of the following expressions represents the diameter of the tree, in inches, when it was years old, assuming and ?
