Mich. Comp. Laws § 141.618: Partial business activity in city; apportionment of net profit.

Sec. 18.

When the entire net profit of a business subject to the tax is not derived from business activities exclusively within the city, the portion of the entire net profit, earned as a result of work done, services rendered or other business activity conducted in the city, shall be determined under either section 19, sections 20 to 24, or section 25.