Length Measurements
Primary 4 · Mathematics
Length Measurements
Lesson 5 of 5
30 min
Let's begin
Niyonzima's family wants to put a fence around their vegetable garden to keep the goats out. The garden is a rectangle 12 m long and 7 m wide. How many metres of fence must they buy? The answer is the perimeter of the garden.
The perimeter of a flat (2D) shape is the total length all the way around its edge. Imagine an ant walking once around the shape: the distance it walks is the perimeter. To find it, add the lengths of all the sides.
The triangle: 5 cm + 7 cm + 9 cm = 21 cm.
A rectangle has two long sides (the length) and two short sides (the width). For Niyonzima's garden:
12 m + 7 m + 12 m + 7 m = 38 m.
A quicker way: add one length and one width, then double: 2 × (12 + 7) = 2 × 19 = 38 m.
Perimeter of a rectangle = 2 × (length + width)
All four sides of a square are the same length, so multiply one side by 4. A goat pen that is a square with sides of 9 m has a perimeter of 4 × 9 = 36 m.
Perimeter of a square = 4 × side
The cost of a fence. Fencing costs 500 Frw for each metre. Niyonzima's garden needs 38 m, so the fence costs 38 × 500 = 19 000 Frw.
A missing side. A rectangle has a perimeter of 30 m and a length of 10 m. Half of the perimeter is one length plus one width: 30 ÷ 2 = 15 m. So the width is 15 − 10 = 5 m.
Explore: build shapes from squares in the Area Builder simulation (Explore screen) and open the Values box to see the perimeter, the distance around each shape.
Tip
Perimeter = the path around. Put your finger on one corner and trace all the way around the shape, adding each side as you pass it. Do not forget any side!
Work each one in your exercise book first. Then tap Show answer to check.
Find the perimeter of a square with sides of 6 cm.
Answer: 24 cm (4 × 6)
Find the perimeter of a rectangle 15 m long and 8 m wide.
Answer: 46 m, because 2 × (15 + 8) = 2 × 23 = 46
Find the perimeter of a triangle with sides 6 cm, 8 cm and 10 cm.
Answer: 24 cm (6 + 8 + 10)
A rectangle has a perimeter of 40 m and a length of 12 m. What is its width?
Answer: 8 m. Half the perimeter is 20 m, and 20 − 12 = 8.
Fencing costs 600 Frw per metre. How much does it cost to fence a square garden with sides of 5 m?
Answer: 12 000 Frw. The perimeter is 4 × 5 = 20 m, and 20 × 600 = 12 000.
What you learned today
Sources and credits
The stories, Rwandan worked examples, diagrams and practice questions were written by Ganzaa.
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