Need help with calculation of lot, please.?

A lot measures 25' x 140' = 3500 square feet. 3500 sq. ft/43,560 sq. ft. per acre = .080349 acreage per lot. Now, I need to know the acreage in the East 15 feet of one of these lots. I am thinking we are dealing with 15 linear feet and not square feet in this case. Thanks.