Which One Doesn’t Belong? — One-Step Equations Activity This Which One Doesn’t Belong? activity is an engaging, low-prep way for students to practice one-step equations while strengthening their reasoning skills. Instead of solving a page of similar problems, students analyze sets of four equations, solve each one, and determine which equation does not belong based on the solution. In each problem set, three equations have the same solution and one equation has a different solution. Stude