Texas Tax Code - TAX § 151.416. Commingled Receipts and Tax

(1) subtracting from the total receipts of the seller the receipts from the sales of items that are exempted or are specifically excluded from the taxes imposed by this chapter to obtain a remainder consisting of the commingled receipts from taxable sales and the taxes collected;  and

(2) dividing this remainder by one plus the sales tax rate expressed as a decimal fraction to obtain a quotient that is the taxable receipts that may be reported under Section 151.410 of this code.