3.MD.5.a
“A square with side length 1 unit, called “a unit square," is said to have "one square unit" of area, and can be used to measure area.”
The unit-square grids under Figures A and B, where each cell is one square unit.
3.MD.5.b
“A plane figure which can be covered without gaps or overlaps by n unit squares is said to have an area of n square units.”
Recording Figure A's area as 12 square units by seeing it covered by 12 grid cells.
3.MD.6
“Measure areas by counting unit squares (square cm, square m, square in, square ft, and improvised units).”
Questions 3 and 7 in Part 1, where areas are found by counting squares before any formula appears.
3.MD.7.a
“Find the area of a rectangle with whole-number side lengths by tiling it, and show that the area is the same as would be found by multiplying the side lengths.”
The Figure A and B work shows counting and multiplying agree: 4 × 3 and 6 × 2 both give the counted 12.
3.MD.7.b
“Multiply side lengths to find areas of rectangles with whole-number side lengths in the context of solving real world and mathematical problems, and represent whole-number products as rectangular areas in mathematical reasoning.”
Part 2's real-world rectangles: 7 ft × 4 ft = 28 sq ft for the lawn and 5 × 5 for the tile.
3.MD.8
“Solve real world and mathematical problems involving perimeters of polygons, including finding the perimeter given the side lengths, finding an unknown side length, and exhibiting rectangles with the same perimeter and different areas or with the same area and different perimeters.”
Every perimeter equation, plus both Part 3 mysteries, where an unknown side is recovered from a given area or perimeter.
