Unit 1: Number Sense
Topic D: Rounding Numbers
We use numbers a lot in our everyday lives. List some of the ways you use numbers.
You may have written money, shopping, time, and counting as part of your answer.
Think about time. Let’s say it takes eight minutes to walk to the bus. If someone asks you how long it takes, you will probably say, “About ten minutes.”
If you buy a sweater that cost $29, you may say, “Oh, it was around thirty dollars.”
How far is it from Vancouver to Prince George? The map says 796 km, but we would probably say, “About 800 kilometres.”
You have just read examples of rounding numbers.
We round numbers for many reasons:
- We may not know the exact number.
- The exact number may not be important for what we are doing.
- We may need a quick way to figure something out.
When you are rounding numbers, use zeros to hold the places at the end of the number. Work through the following examples and exercises carefully. Rounding is an important skill.
Rounding to the Nearest Ten
Here is a short method to round to the nearest ten. When rounding to the nearest ten, do this:
- Underline the tens digit. 83
- Look at the digit following in the ones place. 83
- If the digit in the ones place is less than 5, write a 0 in the ones place. Leave the tens digit as it is.
- 42, rounds to 40 (42 is nearer to 40 than to 50)
- 14, rounds to 10
- 83, rounds to 80
- If the digit in the ones place is 5 or more, write a 0 in the ones place. Add one more ten to the tens place.
- 36, rounds to 40 (36 is nearer to 40 than to 30)
- 25, rounds to 30
- 98, rounds to 100 (one more ten than nine tens is ten tens)
Note: If you are rounding to the nearest ten, single digits are rounded like this:
- 0, 1, 2, 3, 4 all round to 0.
- 5, 6, 7, 8, 9 all round to 10.
A number rounded to the nearest ten will have a zero in the ones place.
The number will end with
0, 10, 20, 30, 40, 50, 60, 70, 80, or 90.
When you round a number, use the sign that means “approximately equal” ≈.
Exercise 1
Round each number to the nearest 10. Check your work using the answer key at the end of the exercise.
- 47 is between tens and tens.
47 is closest to tens.
Rounded number is . - 81 is between tens and tens.
81 is closest to tens.
Rounded number is . - 14 is between tens and tens.
14 is closest to tens.
Rounded number is . - 26 is between tens and tens.
26 is closest to tens.
Rounded number is . - 98 is between tens and tens.
98 is closest to tens.
Rounded number is . - 57 is between tens and tens.
57 is closest to tens.
Rounded number is . - 73 is between tens and tens.
73 is closest to tens.
Rounded number is .
Answers to Exercise 1
- 4 tens, 5 tens
5 tens
50 - 8 tens, 9 tens
8 tens
80 - 1 tens, 2 tens
1 ten
10 - 2 tens, 3 tens
3 tens
30 - 9 tens. 10 tens
10 tens
100 - 5 tens, 6 tens
6 tens
60 - 7 tens, 8 tens
7 tens
70
Exercise 2
Round each number to the nearest ten. Check your work using the answer key at the end of the exercise.
- 22 ≈ 20
- 86 ≈ 90
- 31 ≈
- 96 ≈
- 84 ≈
- 55 ≈
- 8 ≈
- 2 ≈
- 63 ≈
- 49 ≈
- 25 ≈
- 71 ≈
- 38 ≈
- 51 ≈
- 88 ≈
Answers to Exercise 2
- 30
- 100
- 80
- 60
- 10
- 0
- 60
- 50
- 30
- 70
- 40
- 50
- 90
Numbers of any size can be rounded to the nearest ten using the method you have just learned.
[latex]\large \begin{array}{rll} \downarrow && \\ 2\underline{3}8 & \approx & 240 \end{array} \qquad \begin{array}{rll} \downarrow && \\ 8\underline{8}3 & \approx & 880 \end{array}\qquad \begin{array}{rll} \downarrow && \\ 2\underline{9}8 & \approx & 300 \end{array}[/latex]
Exercise 3
Round each number to the nearest ten. Check your work using the answer key at the end of the exercise.
- 424 ≈
- 867 ≈
- 499 ≈
- 132 ≈
- 278 ≈
- 617 ≈
- 208 ≈
- 851 ≈
- 124 ≈
- 576 ≈
- 315 ≈
- 742 ≈
- 397 ≈
- 952 ≈
- 639 ≈
Answers to Exercise 3
- 420
- 870
- 500
- 130
- 280
- 620
- 210
- 850
- 120
- 580
- 320
- 740
- 400
- 950
- 640
Exercise 4
For each problem, round the numbers to the nearest ten. Check your work using the answer key at the end of the exercise.
Example:
Mei Ling has just moved into a new apartment. She bought the following items. Round each amount to the nearest ten.
| Item | Cost | Rounded to nearest ten |
|---|---|---|
| Towels | $14 | $10 |
| Dishes | $32 | $30 |
| Saucepan | $43 | $40 |
| Microwave | $109 | $110 |
| Carving knife | $18 | $20 |
- Akkul walked 12 kilometres on Monday, 26 kilometres on Tuesday and 6 kilometres on Wednesday. Round each number to the nearest ten.
Day Number Rounded Number Monday 12 Tuesday 26 Wednesday 6 - Werner is a keen bird watcher. On Monday, he saw 57 birds, on Tuesday he saw 124 birds, on Wednesday he saw 31 birds and on Thursday he saw 75 birds. Round each number to the nearest ten.
Day Number Rounded Number Monday 57 Tuesday 124 Wednesday 31 Thursday 75 - Jamir drove 678 kilometres, 493 kilometres, 387 kilometres and 914 kilometres in one week. Round each mileage to the nearest ten.
Day Kilometres Rounded Number #1 678 #2 493 #3 387 #4 914
Answers to Exercise 4
- 10, 30, 10
- 60, 120, 30, 80
- 680, 490, 390, 910
Topic D Self-Test
Mark /12 Aim 10/12
- Round your answer to the nearest ten. (8 marks)
- 47 ≈
- 123 ≈
- 4 ≈
- 945 ≈
- 329 ≈
- 481 ≈
- 865 ≈
- 916 ≈
- Round each number to the nearest ten. (4 marks)
Mary scored 78, 91, 79, 67 and 102 on her arithmetic test. Round her scores to the nearest ten.Score Rounded Score 78 91 79 67 102
Answers to Topic D Self-Test
-
- 50
- 120
- 0
- 950
- 330
- 480
- 870
- 920
-
- 80, 90, 80, 70, 100