Mont. Code Ann. § 15-68-411

If a purchaser uses a nontaxable transaction certificate with respect to the purchase of fungible goods and commingles these goods with fungible goods that were not purchased with a certificate but that are of such similarity that the identity of the goods in the commingled mass cannot be determined, sales from the mass of commingled goods are considered to be sales of the goods purchased with the certificate until the quantity of commingled goods sold equals the quantity of goods originally purchased under the certificate.