Approximation: rounding and truncating
Approximate a number to the nearest 10, 100, 1 or 0.1 with two methods, rounding and truncating, tell which one is rounded, approximate a fraction and estimate a product.
- France: grade 5
- Iran: grade 6
Guide
To approximate a number we decide how accurate we want to be: "to the nearest 0.1" means to the tenths digit, "to the nearest 100" means to the hundreds. In the table that column has a box and the next column, which decides, is dashed.
To round (rounding), look at the dashed digit. If it is 5 or more, the digit before goes up by one; if it is less than 5, it stays. 4.78 rounded to 0.1 is 4.8.
To truncate, we just drop the later digits and add nothing. 4.78 truncated to tenths is 4.7. Rounding gives the nearer number. A truncated approximation is always less than or equal to the true number.
To approximate a fraction, divide the numerator by the denominator and then approximate the answer. 27 is about 0.2857, and to 0.1 that is 0.3.
To estimate a product, round each number: 482 × 31 is about 500 × 30. Measuring tools also have accuracy: a bathroom scale shows a person's mass to the nearest kilogram.