(1) Any corporation having income from business activity which is taxable both within and without this state shall allocate and apportion its net income as provided in this part.
(2) A corporation engaged in a unitary business within and without Montana must apportion its apportionable income as provided for under 15-31-305. A business is unitary when the operation of the business within the state is dependent upon or contributory to the operation of the business outside the state or if the units of the business within and without the state are closely allied and not capable of separate maintenance as independent businesses.
(3) A corporation not engaged in a unitary business must allocate its apportionable income by means of separate accounting methods, provided its books and records are so kept that the income and expenses attributable to business operations within the state can be properly segregated from total income and expense. If the corporation's books and records do not permit such proper segregation, its apportionable income must be apportioned according to the provisions of 15-31-305.