65
Estimating values (practice)
Textbook: Part 2, lesson 65, pp. 81–83
GoalEstimating the lower and upper bounds of a product.
New words
factor · ko‘paytuvchiproduct · ko‘paytmalower bound · quyi chegaraupper bound · yuqori chegara
Explanation
To estimate a product we multiply round numbers smaller than each factor to get the lower bound and round numbers larger than each factor to get the upper bound. For 23 × 57 we get 20 × 50 = 1 000 and 30 × 60 = 1 800, so 1 000 < 23 × 57 < 1 800. Estimating helps find gross errors in a calculation.
Worked examples
46 × 27: lower 40 × 20 = 800, upper 50 × 30 = 1 500; exact 1 242.
For 67 × 48: 60 × 40 = 2 400 and 70 × 50 = 3 500; 2 016 is not in this range.
Class activity
“Spot the error”: the teacher writes wrong products and children find the mistake with bounds.
Practice
1
What is the lower bound (40 × 20) of the product 46 × 27?
800
2
What is the upper bound (50 × 30) of the product 46 × 27?
1500
3
Someone says 67 × 48 = 2 016. The lower bound 60 × 40 is 2 400. Can this be right?
No, 2 016 < 2 400
4
A room is 4 m 90 cm wide and 6 m 10 cm long. Estimate its area as 5 m × 6 m. How many m²?
30