127
Properties of multiplication
Textbook: Part 4, lesson 118, pp. 6–7
GoalCalculate products of three or four factors in a handy way using the properties.
New words
handy way · qulay usulpairing · juftlashexpression · ifodain total · jami
Explanation
We can swap and group the factors to make the work easier. The handiest pair is one whose product is 10: 2 × 5 = 10. 2 × 4 × 5 = (2 × 5) × 4 = 10 × 4 = 40. It works the same with four factors: 2 × 3 × 2 × 5 = (2 × 5) × (3 × 2) = 10 × 6 = 60.
Worked examples
5 × 2 × 4: first 5 × 2 = 10, then 10 × 4 = 40.
3 × 5 × 2 × 2 = (5 × 2) × (3 × 2) = 10 × 6 = 60.
Class activity
“Find the ten-pair”: write an expression such as 2 × 3 × 5 × 4 on the board. Children circle two factors whose product is 10 and call out the result. The fastest correct pair wins.
Practice
1
2 × 7 × 5 = ?
70
2
4 × 6 × 2 = ?
48
3
2 × 3 × 2 × 5 = ?
60
4
A school has 4 classes. Each class has 5 rows with 2 desks in each row. How many desks are there in all?
40