SME
Grade 3 Math · Problem type
Area of composite figures
Use this to find the area of an L-shape made of two rectangles.
Diagram
15 + 8 = 23 cm²
This is an example. Type your own values to start a new problem.
Left width
×
Left height
+
Right width
×
Right height
=
Left area
+
Right area
=
Total area (cm²)
Formulas
3 × 5 = 15
Left width × left height = left area
4 × 2 = 8
Right width × right height = right area
15 + 8 = 23
Left area + right area = total area
Assumptions
•
Cut the shape into two rectangles that don’t overlap.
•
Find each rectangle’s area, then add them.
•
Each square is 1 square unit.
Step-by-step
WE KNOW
Left width: 3 cm
Left height: 5 cm
Right width: 4 cm
Right height: 2 cm
FIND
left area, right area, total area
Step 1 · Find left area
3 × 5 = ?
Left width × left height = left area
Multiply the width by the height.
Count by 5s, 3 times: 5, 10, 15 → 15
Left area = 15 cm²
Step 2 · Find right area
4 × 2 = ?
Right width × right height = right area
Multiply the width by the height.
Double 2: 2 + 2 = 4
Double again: 4 + 4 = 8
Right area = 8 cm²
Step 3 · Find total area
15 + 8 = ?
Left area + right area = total area
Add the left area and the right area.
15 + 5 = 20
20 + 3 = 23
Total area = 23 cm²
Check
Put the numbers back in. Both sides should match.
3 × 5 = 15 ✓
4 × 2 = 8 ✓
15 + 8 = 23 ✓
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Area and the distributive property
Next lesson →
Area of a shape with a corner cut out
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Related lessons
Area and multiplication
Main lesson
Find an unknown side from the area
Use this when you know the area and one side: “24 cm², 6 cm long. How wide?”
Area and the distributive property
Use this to split a rectangle into two and add the areas: 6 × 8 = 6 × 5 + 6 × 3.
Area of a shape with a corner cut out
Use this for an L-shape: the whole rectangle take away the corner cut out.