145
Multiplying round numbers
Textbook: Part 4, lesson 135, pp. 42–43
GoalUse multiplying round numbers with handy methods and in perimeter and area problems.
New words
perimeter · perimetrarea · yuz (maydon)mass · massahandy method · qulay usul
Explanation
We use the grouping property: (2 × 3) × 50 = 6 × 50 = 300. Everyday problem: if a factory makes 40 kg of sweets a day, in 5 days it makes 40 × 5 = 200 kg. If a rectangular field is 50 m wide and 100 m long: its perimeter is 2 × (50 + 100) = 300 m and its area is 50 × 100 = 5000 m².
Worked examples
(3 × 8) × 5 = 3 × (8 × 5) = 3 × 40 = 120.
50 × (2 × 7) = 50 × 14 = 700.
Class activity
“Field and fence”: each pair draws a rectangle 5 squares wide and 10 squares long (imagine 1 square = 10 m) and finds the perimeter and area by multiplying round numbers.
Practice
1
40 × 5 = ?
200
2
(2 × 3) × 50 = ?
300
3
50 × (2 × 7) = ?
700
4
A factory makes 40 kg of sweets a day. How many kilograms does it make in 2 weeks (10 working days)?
400
5
A field is 50 m wide and twice as long as it is wide. What is its perimeter in metres?
300
6
What is the area of this field in m²?
5000