The Taxpayer operated a day care facility in New Mexico from 1995 to 2001. He provided meals and day care services some of which were paid for by government programs administered by the New Mexico Children, Youth and Families Department (CYF). The Taxpayer paid gross receipts tax on receipts paid directly by parents, but mistakenly deducted all of his receipts from CYF. Due to this error, the Taxpayer underreported his gross receipts tax by more than 25 percent. In 2001, following a Department audit, the Taxpayer was assessed for gross receipts tax and accrued interest. No penalty was assessed. The Taxpayer protested the assessment of interest and made the following arguments: (1) CYF originally failed to notify him that he was liable for gross receipts tax on the payments he received; (2) the Department waited too long to audit the Taxpayer and unreasonably chose to extend its audit to six years instead of the normal three-year period, and (3) the 15 percent rate used to calculate interest is an unfair penalty on taxpayers. Held: (1) the Taxpayer, not CYF or the Department, was responsible for determining his tax liability to the state; (2) pursuant to Section 7-1-18(D) NMSA 1978, the Department was required to extend the audit back six years once it was determined that the Taxpayer underreported his receipts by more than 25 percent; and (3) the Taxpayer was late in paying gross receipts taxes due to the state, and interest was properly assessed pursuant to Section 7-1-67 NMSA 1978. Protest denied.