But actual how it was shared?
Let's assume for a moment, the Distance between Westville and Eastville is 50 miles.
In this case, John drives only 40 miles while Mary drives 10 + 50 = 60 miles. For any distance beyond 50 miles, Mary drives west equal distance as John drives west.
So in any case the difference will remain same of 20 miles. So Mary drives the farthest by 20 miles.