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Number Trail Grade 4 / Week 11 / Wednesday
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Week 11 · Measure and convert

Wednesday

Area and perimeter formulas
// Units, area, perimeter and line plots
⏱ about 15 min

Wednesday: Area and perimeter formulas

Hazel the Hedgehog wants to build a garden bed for her pumpkins.

Bennie the Bear marks out a rectangle 8 feet long and 6 feet wide.

"I need soil to cover the inside," says Hazel. "And a little fence to go around it."

"Soil covers the area," says Bennie. "The fence goes around the perimeter."

Hazel taps her pencil. "Counting every square foot would take a long time."

"Then let us use formulas," says Bennie. "They are shortcuts that always work for rectangles."

Learn it: two formulas

A rectangle 8 feet long and 6 feet wide

Area = length × width.

Area = 8 × 6 = 48 square feet. Hazel needs soil for 48 square feet.

Perimeter = 2 × length + 2 × width.

Perimeter = 2 × 8 + 2 × 6 = 16 + 12 = 28 feet of fence.

We can also find a missing side.

A rug has an area of 48 square feet and a length of 8 feet.

Think 8 × ? = 48. The width is 6 feet.

AREA
  • Read the question.
  • Tap your answer.
A rectangle 8 ft long and 6 ft wideWhat is the area in square feet?
A rectangle 12 m long and 5 m wideWhat is the area in square meters?
A rectangle 15 cm long and 4 cm wideWhat is the area in square centimeters?
A rectangle 25 m long and 3 m wideWhat is the area in square meters?
PERIMETER
  • Read the question.
  • Tap your answer.
A rectangle 8 ft long and 6 ft wideWhat is the perimeter in feet?
A rectangle 12 m long and 5 m wideWhat is the perimeter in meters?
A rectangle 20 cm long and 9 cm wideWhat is the perimeter in centimeters?
A rectangle 7 m long and 7 m wideWhat is the perimeter in meters?
FIND THE MISSING SIDE
  • Read the question.
  • Tap your answer.
8 × ? = 48
? × 9 = 72
2 × 10 + 2 × ? = 30
A rugA rug has an area of 54 square feet. It is 9 feet long. How many feet wide is it?
FIND A MISSING WIDTH
  • Think about how Bennie found the width of the rug.
  • Tap the steps in order.
1First
2Next
3Then
4Last
SentenceTrue or false?
8 × 6 = 48?
2 × 8 + 2 × 6 = 28?
2 × 12 + 2 × 5 = 60?
Area is measured in square units.?
Perimeter is length times width.?
WHY THIS EXERCISE2 × 12 + 2 × 5 is 24 + 10, which is 34. Perimeter adds all the sides; area multiplies.
Try it
Measure the length and width of a book or a table in inches.
Find the area with length × width.
Find the perimeter with 2 × length + 2 × width.
Which number is bigger for your shape?
Draw a garden bed 9 feet by 4 feet. Write its area and perimeter.

Super work! Tomorrow Chris tells a story about Hazel's feathers.

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