Problem
Both equal sides are 21 cm, so add all three sides to get the perimeter.
Grade 4 students find a perimeter by adding the side lengths around the shape.
4.MD.A.3Identify SubproblemsThe equilateral triangle must have the same 57 cm perimeter, and all three of its sides are equal, so divide the perimeter by 3.
Equal sides means simply splitting the total length into 3 equal parts, a Grade 4 division idea.
4.G.A.2Identify SubproblemsEach side of the equilateral triangle equals the shared perimeter split evenly into three parts.
The equilateral triangle's perimeter is just its three sides joined end to end, so those sides together make up the whole perimeter.
Tracing the boundary cuts it into three side-pieces with no gaps and no overlaps, and pieces like that always add back to the whole.
Because all three sides of an equilateral triangle are the same length, the perimeter is one side length counted three times.
Three equal groups of the same side length is that length added over and over, which is exactly what multiplying by three means.
Since the perimeter is a known total made of three equal sides, splitting that total into three equal parts hands back one side.
Dividing by three reverses multiplying by three, so it undoes the tripling and recovers the single side.
Each side of the equilateral triangle is 19 cm — the unit stays centimeters throughout.
Keeping the unit (cm) consistent confirms the answer is a length, not a count.
4.MD.A.1Analyze The UnitsThis only needs Grade 4 perimeter-adding and equal-sharing you already know!