This activity is great because it forces your students to analyze another person's work in order to decide if it is worked out correctly or not. If it's incorrect, then the student must fix the mistake. This activity is great to use individually or with a group of students. I usually have them work in groups, and they must decide together if it's right or wrong. Students bring up some great mathematical discussions, this way!
Check out more of my "Analyzing Activities":
Analyzing Angle Relationships
Analyzing Multi-Step Equations