“ (3) The total profit margin for a prescribed accounting period shall be the amount (if any) by which the total selling price calculated in accordance with paragraph (4) below, exceeds the total purchase price calculated in accordance with paragraph (5) below. (4) For the purposes of paragraph (3) above the total selling price shall be calculated by aggregating for all goods sold during the period the prices (calculated in accordance with article 12(5) or (6) above as appropriate) for which they were sold. (5) For the purposes of paragraph (3) above the total purchase price shall be calculated by aggregating for all goods obtained during the period the prices (calculated in accordance with article 12(5) above) at which they were obtained and adding to that total the amount (if any) carried forward from the previous period in accordance with paragraph (6) below; (6) If in any prescribed accounting period the total purchase price calculated in accordance with paragraph (5) above exceeds the total selling price, the excess amount shall be carried forward to the following prescribed accounting period for inclusion in the calculation of the total purchase price for that period. ”