This product is a sort with 30 cards. Only a few cards are pictured in the sample. Some of the cards are true and some of the cards are false. For example, a true card would be 300 + 20 + 5 = 325. A false card would be 300 20 5 = 325. It is false because 300 20 5 is really 300,205, which is not equal to 325. These are errors that I took from my own students' work.
This product could be used as a center or you could take a card and use it for error analysis.