In this algebra lesson...

the students will move toward an algebraic equation that will represent many situations, but the teacher begins with something concrete: a 10 by 10 grid. She gives students a problem to mentally solve for which there are many approaches to solution: how many squares are in the border? After sharing their answers with a partner, the teacher starts the whole class discussion by getting the answer on the table. The answer is not the focus. The two easy mistakes to make (4x10=40 and (2x10)+(2x9)=38) are also dispensed with up front. So the focus of the class will be on alternative strategies for getting the same (correct) answer.