Problem
Viewed from below, 9 of the 11 cubes touch the floor; those 9 squares form an L (a row of 6 with a row of 3 under its left end).
Telling apart the flat floor shape (2D) from the stacked cubes (3D) is the kindergarten idea of two- vs three-dimensional shapes.
K.G.A.3Visualize Spatial RelationshipsOn grid paper the L fits inside a 6 cm by 2 cm rectangle with a 3 cm by 1 cm notch cut from the bottom-right.
Sketching the flat outline turns a 3D puzzle into a simple polygon-perimeter problem at the Grade 3 level.
3.MD.D.8Draw A DiagramWalk the outline, adding 1-cm edges: 6 across the top, 1 down, 3 left, 1 down, 3 left, 2 up — summing to the perimeter.
For an L made of two rectangles, the inner step lengths slide together so the perimeter equals that of the 6 by 2 bounding rectangle: 2 x (6 + 2) = 16.
4.MD.A.3Identify SubproblemsWalking the outline of the L-shaped floor figure and adding its 1-cm edges gives a perimeter of 16 cm.
The perimeter is the whole distance around the figure, and that outline is one closed path cut into straight runs that meet end to end with no gaps or overlaps, so its total length is those run lengths added together.
Each straight run lies along the sides of the unit squares, and every square side is exactly 1 cm, so a run's length is simply how many of those 1-cm sides lie along it.
Laying that many 1-cm sides in a straight line makes that many equal lengths of 1 cm, which measures that many centimeters.
Look at the solid from underneath, draw the flat shape it sits on, then add the outside edges - an L-shape's perimeter matches its bounding box!