(2)Computation of tax. If an article subject to tax at the rate of 10 percent is sold for $100 and an additional item of $10 is billed as tax, $100 is the taxable selling price and $10 is the amount of tax due thereon. However, if the article is sold for $100 with no separate billing or indication of the amount of the tax, it will be presumed that the tax is included in the $100, and a computation will be necessary to determine what portion of the total amount represents the sale price of the article and what portion represents the tax. The computation is as follows: Taxable sale price = sale price including tax/100 + rate of tax.
Thus, if the tax rate is 10 percent and the sale price including tax is $100, the taxable sale price is $90.91 (that is, $100 divided by (100 + 10)), and the tax is 10 percent of $90.91, or $9.09.