Answer
2.5 or $\frac{f}{2.5}$
Work Step by Step
The front-to-back length of the eyeball is the image distance for faraway objects, i.e., it is the focal length.
Use the relationship on page 715 that defines the f-stop number.
$$f-stop=\frac{f}{D}=\frac{20mm}{8.0mm}=2.5$$
