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.