The business income tax base and modified gross receipts tax base of "a taxpayer whose business activities are subject to tax both within and outside of this state shall be apportioned to this state" by multiplying the business income tax base and modified gross receipts tax base by the sales factor. MCL 208.1301 (emphasis added). The sales factor is a fraction, "the numerator of which is the total sales of the taxpayer in this state during the tax year and the denominator of which is the total sales of the taxpayer everywhere during the tax year." MCL 208.1303(1). Sales are sourced to Michigan or elsewhere under MCL 208.1305.

"Taxpayer" means "a person or a unitary business group liable for a tax, interest, or penalty under this act." MCL 208.1117 (emphasis added). In other words, apportionment is not calculated separately for each member of the unitary business group but on the combined tax bases as calculated under MCL 208.1201 and 208.1203. There is no need to allocate the apportioned tax base of the unitary business group back to the members of the unitary business group.