Problem
Walk clockwise from the top-left: the top edge is segment 1, the right edge is segment 2 — 2 so far.
Recognizing each straight side as one line segment is exactly the Grade 4 idea of identifying line segments in a figure.
4.G.A.1Draw A DiagramTrace clockwise across the notched bottom — floor, up-ledge-up-across-down-ledge-down, floor — adding 9 straight stretches (3–11).
Listing each straight stretch in walking order keeps every up/down/across step accounted for exactly once.
4.G.A.1Make A Systematic ListWalking the notched bottom of the figure from where the right floor meets it to where the left floor leaves it, it is made of exactly nine straight line segments.
A line segment here is one straight stretch between two neighboring corners, and a fresh stretch begins every time the outline turns, so the segments are simply the separate straight pieces the bottom is cut into.
Those pieces sit end to end with no gap between them and no piece lying on top of another, so together they rebuild the whole bottom and counting the pieces accounts for the whole bottom exactly.
Tracing the bottom once, in order, and tallying each stretch as you reach it lines up every stretch with one step of the walk, so no stretch is skipped and none is counted twice.
When each stretch is matched with exactly one step of the walk, the number of tallies is the same as the number of stretches — no more and no less.
The left edge rises back to the top-left corner — the last segment (12) closes the outline.
Counting the final closing side is simple addition, and the walk returning to its start confirms the loop is complete.
3.OA.D.9Draw A DiagramWalk the outline like a pencil and count every time it turns - each straight stretch between turns is one line segment!