Select rows and columns, count their overlap, and connect fractional side lengths to square units.
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Set 3 rows with 2 selected and 4 columns with 3 selected. What fraction of the unit square is in both selections?
The square has 3 × 4 = 12 equal cells. The overlap covers 2 × 3 = 6 cells: 6/12 = 1/2.
Watch for: Counting the union of the shaded rows and columns instead of the overlap.
Set 4 rows with 3 selected and 2 columns with 1 selected. What is 3/4 of 1/2?
The overlap is three of eight equal cells. Taking three quarters of one half gives less than one half.
Watch for: Assuming multiplication must make a number larger.
Set 2 rows with 1 selected and 2 columns with 1 selected. The whole square has side length 1 meter. What is the overlap area in square meters?
The overlap has side lengths 1/2 m and 1/2 m. Its area is 1/4 square meter, not the sum of its side lengths.
Watch for: Adding side lengths to calculate area.