* Prime Meridian: The Prime Meridian is at 0 degrees longitude.

* Halfway between 0 and 80 degrees: The halfway point is 40 degrees.

* West of the Prime Meridian: This means we are in the Western Hemisphere.

Therefore, the line of longitude for a place west of the Prime Meridian and halfway between 0 and 80 degrees longitude is 40 degrees West (40°W).