Mystery Perimeters is Common Core Aligned for 3rd grade, and it also makes a great review for grades 4 and 5, especially for those students who need a concrete, hands-on lesson to understand how to find unknown polygon side lengths. This lesson includes a teacher direction page followed by ready-to-use printables and activity pages for demonstrating the concept.
Mystery Perimeters is a sample from my complete ebook for perimeter concepts, Exploring Perimeters. This lesson uses inches for measurement, but the complete lesson in Exploring Perimeter also includes a metric variation that uses centimeters.
To preview the entire book, click the link below:
perimeter lessons, finding perimeter, missing side perimeter, unknown length perimeter, Laura Candler, 3.MD.D.8