A colleague quietly pointed out the error to me. Embarrassed, but happy that the colleague had the courage to let me know the error, I turned the mistake into a learning opportunity. First I relayed to the students that there was an error in the solution, and how happy I was that my colleague was willing to point it out. Then I demonstrated the error using chart paper and a similar area problem. Students mapped an area in feet, then re-marked the grid with yards. We created a scale and compared it to the problem we incorrectly solved the day before.
This error also led to next week's assignment which will be to design a floor plan, playground, or park using grid paper. As part of the design process students will identify the square footage of the design as well as the perimeter. They will also identify the specific features of the plan with words, colors, and perhaps other design features. It will be a good way to review the lesson learned today as well as area and perimeter. Onward.