144
Multiplying round numbers
Textbook: Part 4, lesson 134, pp. 40–41
GoalMultiply round numbers: drop the zeros, multiply, then write the zeros back.
New words
round number · yaxlit sonzeros · nollarproduct · ko‘paytmaten · o‘nlik
Explanation
40 × 3: 4 tens × 3 = 12 tens = 120. Rule: mentally drop the zeros at the ends of the numbers, multiply what is left, then write all the dropped zeros after the product. 30 × 80: 3 × 8 = 24, two zeros were dropped, so the answer is 2400. 300 × 5: 3 × 5 = 15, add two zeros – 1500.
Worked examples
70 × 80: 7 × 8 = 56, two zeros → 5600.
2 × 80 × 5 = (2 × 5) × 80 = 10 × 80 = 800.
Class activity
“Counting zeros”: cards show 30 × 50, 20 × 300… Children first say how many zeros will be dropped, then find the main product and add the zeros to announce the full answer.
Practice
1
40 × 3 = ?
120
2
300 × 5 = ?
1500
3
30 × 50 = ?
1500
4
70 × 80 = ?
5600
5
2 × 80 × 5 = ?
800
6
3 classes of 20 pupils each came to breakfast. How many pupils had breakfast in all?
60