Problem
The 3 congruent squares share the 30 cm bottom equally, so divide 30 by 3 to get the side of one square.
Sharing a total length equally among equal parts is just division — a 3rd-grade idea, no formulas needed.
3.OA.A.2Identify SubproblemsThe base runs the whole 30 cm top, and equilateral means the two slanted sides are 30 cm too.
Equilateral means all sides match, so once I know the base I know the slanted sides too.
3.MD.D.8Identify SubproblemsOutside are two 30 cm slants, two 10 cm square sides and the 30 cm bottom; the seams stay inside.
Perimeter is the path around the outside, so I only collect the edges I would actually walk along.
3.MD.D.8Draw A DiagramThe perimeter of ABCDE is the two slanted triangle sides together with the left, bottom, and right edges of the row of squares.
The perimeter is the path around the outside, so only edges lying on that outer boundary belong to it and the inner edges where the squares meet do not.
Each slanted side is 30 cm, because an equilateral triangle has all three sides the same and its base runs the whole top of the row.
That top is three equal squares set side by side, so its length is three groups of one square's side.
Add the five outside edge lengths to get the perimeter.
Adding up the lengths of every outside edge is exactly what perimeter means.
3.MD.D.8Identify SubproblemsTo find the perimeter of a shape made of pieces, just walk around the outside and add up only the edges you would step along.