This activity includes 4 problems worked out by substitution. Each problem has an error in it (problem 2 actually has two errors). I used mistakes I saw my students commonly making. The activity asks students to find the error, explain the error, then rework the problem correctly. This is a really good group work assignment as it will get students talking about common errors they make. Each group could be assigned their own problem to correct, rework, and then present. It could also be used as an individual assignment or homework.