Some calculus

*might *make it a bit clearer, if you know that the average value of a function

on an interval

is

.

Suppose at a given time

the Sun is a distance

miles from the Earth, and further suppose that the Earth revolves around the Sun exactly once between times

and

.

Then any light ray hitting the Earth at a time

will have traveled for

seconds. As

should be a continuous function of

we may determine the average time to be

Carrying the units through, the answer is in seconds, and this agrees with angel.white's solution.