In groups of four, students work on place-mats 1-11 to complete each problem. After they find their solutions they add them together to find the sum. The teacher can just check the sum to see if the group is correct. If not, then the students need to inspect each other's work to find the error. The questions increase in difficulty as the card number increases.
I laminated the mats so students can write on them and erase with markers.