A. As to business passing through, into or out of this state, the gross receipts of any such corporation exercising its franchises or charters in this state, for the purpose of this Subchapter, shall be determined as provided in Subsection B of this Section.
B. The gross receipts of such corporation over the whole extent of the mileage over which such business is done, within, through and without the state, shall be divided by the total number of miles over which such business is done to ascertain the average gross receipts per mile, and the gross receipts in this state shall be taken to be the average gross receipts per mile, multiplied by the number of miles over which such business is done in this state. For the purposes of determining gross receipts, the cost of gas purchased for resale is deductible.