The third-grade teacher in Gwinnett County, Ga., who assigned math homework that asked questions about slavery and beatings, has resigned and apologized for the incident.

Luis Rivera, who taught at Beaver Ridge Elementary School since 2008, wrote a letter to school officials saying he "cannot apologize enough."

Rivera resigned during the course of an investigation into the incident.

On Wednesday, district spokeswoman Sloan Roach released a statement regarding the resignation saying, "The principal will move forward immediately to fill the vacancy created by this resignation. As this is a personnel matter, the district will not elaborate further."

Rivera assigned math homework that included the question, "Each tree had 56 oranges. If eight slaves pick them equally, then how much would each slave pick?"

Another math problem read, "If Frederick got two beatings per day, how many beatings did he get in one week?"

Another question asked how many baskets of cotton Frederick filled.

Christopher Braxton told ABC News affiliate WSB-TV in Atlanta that he couldn't believe the assignment his 8-year-old son brought home from of Beaver Ridge Elementary school in Norcross.

"It kind of blew me away," Braxton said. "Do you see what I see? Do you really see what I see? He's not answering this question."

"I was furious," Braxton said.

"This outrages me because it just lets me know that there's still racists," said Stephanie Jones, whose child is a student at the school.

"Something like that shouldn't be imbedded into a kid of the third, fourth, fifth, any grade," parent Terrance Barnett told WSB-TV. "I'm having to explain to my 8-year-old why slavery or slaves or beatings are in a math problem. That hurts."

"In this one, the teachers were trying to do a cross-curricular activity," Roach said.

Roach said the teachers were attempting to incorporate social studies into math problems.

"We understand that there are concerns about these questions, and we agree that these questions were not appropriate," she said.

ABC News affiliate WSB-TV contributed to this report.