This is a GREAT rigorous activity as an extension for your students that already know how to add big numbers. This follows greatly with 3rd and 4th grade SOLs. Students act as they are the teacher and they "grade" the math paper, deciding which have the correct and incorrect answers.
The way the questions are being asked is exactly how they will see it on the SOL and really make the students think.
The students think it is so fun to be the teacher and get to grade someones paper!