GoalReview area and perimeter, elapsed time, estimating and three-digit subtraction.
New words
area · the number of square units inside a shapeperimeter · the distance around a shapeelapsed time · how much time passes from a start to an end
Explanation
For a rectangle, area = length × width in square units, and perimeter = the sum of all four sides in plain units. A rectangle 6 cm by 4 cm has area 6 × 4 = 24 square cm and perimeter 6 + 4 + 6 + 4 = 20 cm. An L-shaped room can be cut into rectangles; add their areas. For elapsed time, jump to the next hour, then by hours, then by the last minutes: from 2:45 p.m. to 5:15 p.m. is 15 minutes + 2 hours + 15 minutes = 2 hours 30 minutes. To estimate, round to the nearest ten or hundred. To subtract, count up or regroup, and check by adding: 702 − 345 = 357 because 357 + 345 = 702.
Worked examples
A rectangle is 9 cm long and 5 cm wide. Its area is 9 × 5 = 45 square cm. Its perimeter is 9 + 5 + 9 + 5 = 28 cm.
This room is made of two rectangles. Cut it at the dotted line: the bottom part is 7 by 3 = 21 and the top part is 3 by 2 = 6, so the area is 27 square units. Add the sides for the perimeter: 7 + 3 + 4 + 2 + 3 + 5 = 24 units.
A training session starts at the time on the clock, 2:45 p.m., and ends at 5:15 p.m. 2:45 to 3:00 is 15 minutes, 3:00 to 5:00 is 2 hours and 5:00 to 5:15 is 15 minutes. It lasts 2 hours 30 minutes.
Class activity
Garden challenge. A rectangular garden has a fence of 24 m all the way round, with whole-metre sides. Find as many different rectangles as you can, for example 5 m by 7 m. Work out the area of each and say which one has the greatest area. Then write the time you start and finish this activity and find how long it took.
Practice
1
A rectangle is 12 cm long and 7 cm wide. Find its area and its perimeter.
Area: 12 × 7 = 84 square cm. Perimeter: 12 + 7 + 12 + 7 = 38 cm.
2
Class starts at the time on the clock (a.m.) and ends at 11:20 a.m. How long is the class?
The clock shows 8:50 a.m. 8:50 to 9:00 is 10 minutes, 9:00 to 11:00 is 2 hours and 11:00 to 11:20 is 20 minutes. The class lasts 2 hours 30 minutes.
3
A square courtyard has sides of 9 m. Find its area and perimeter.
