5
Rounding natural numbers
Textbook: Part 1, lesson 5, pp. 22–23
GoalRound numbers to tens, hundreds and thousands; tell estimating and rounding apart.
New words
rounding · yaxlitlashestimating · chamalasha round ten · o‘nlikapproximately equal (≈) · taxminan teng (≈)
Explanation
When we estimate a number, we take either the smaller or the larger nearby round number (ten, hundred, thousand). When we round, we take the round number that is nearest to it. To do that, place the number between two neighbouring round numbers and see which is closer. If the number is equally far from both, take the larger one: 65 ≈ 70. A rounded number is not equal to the original, so we write the sign “≈”, read “is approximately equal to”.
Worked examples
Round 3 462 to tens: 3 460 < 3 462 < 3 470, and 3 462 − 3 460 = 2, 3 470 − 3 462 = 8. Since 2 < 8, 3 462 ≈ 3 460.
Round 8 450 to hundreds: 8 400 < 8 450 < 8 500; the number is 50 away from each. For equal distance we take the larger: 8 450 ≈ 8 500.
Class activity
“Market prices”: pupils round the prices of 4 products (for example 2 380 and 7 940 so‘m) to hundreds and say roughly how much must be paid.
Practice
1
Round 8 462 to hundreds.
8 500
2
Round 15 to tens.
20
3
Round 36 249 to thousands.
36 000
4
Why is 450 rounded to 500 when rounding to hundreds?
450 is 50 away from both 400 and 500; in such a case the larger hundred is taken.