1.5: Topic E- Rounding Numbers

Topic E: Rounding Numbers

We use numbers a lot in our everyday lives. List some of the ways you use numbers.

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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:

1. Underline the tens digit. 83
2. Look at the digit following in the ones place. 83
3. 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
4. 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

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” \(\approx\).

Exercise 1

Round each number to the nearest 10. Check your work using the answer key at the end of the exercise.

1. 47 is between _____ tens and _____ tens.
   47 is closest to _____ tens.
   Rounded number is _____ .
2. 81 is between _____ tens and _____ tens.
   81 is closest to _____ tens.
   Rounded number is _____ .
3. 14 is between _____ tens and _____ tens.
   14 is closest to _____ tens.
   Rounded number is _____ .
4. 26 is between _____ tens and _____ tens.
   26 is closest to _____ tens.
   Rounded number is _____ .
5. 98 is between _____ tens and _____ tens.
   98 is closest to _____ tens.
   Rounded number is _____ .
6. 57 is between _____ tens and _____ tens.
   57 is closest to _____ tens.
   Rounded number is _____ .
7. 73 is between _____ tens and _____ tens.
   73 is closest to _____ tens.
   Rounded number is _____ .

Answers to Exercise 1

1. 4 tens, 5 tens
   5 tens
   50
2. 8 tens, 9 tens
   8 tens
   80
3. 1 tens, 2 tens
   1 ten
   10
4. 2 tens, 3 tens
   3 tens
   30
5. 9 tens. 10 tens
   10 tens
   100
6. 5 tens, 6 tens
   6 tens
   60
7. 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.

1. 22 ≈ 20
2. 86 ≈ 90
3. 31 ≈ _____
4. 96 ≈ _____
5. 84 ≈ _____
6. 55 ≈ _____
7. 8 ≈ _____
8. 2 ≈ _____
9. 63 ≈ _____
10. 49 ≈ _____
11. 25 ≈ _____
12. 71 ≈ _____
13. 38 ≈ _____
14. 51 ≈ _____
15. 88 ≈ _____

Answers to Exercise 2

3. 30
4. 100
5. 80
6. 60
7. 10
8. 0
9. 60
10. 50
11. 30
12. 70
13. 40
14. 50
15. 90

Numbers of any size can be rounded to the nearest ten using the method you have just learned.

\[\large \begin{array}{rll} \downarrow && \\ 2\underline{3}8 & \approx & 240 \end{array} \qquad \begin{array}{rll} \downarrow && \\ 8\underline{8}3 & \approx & 880 \end{array}\qquad \begin{array}{rll} \downarrow && \\ 2\underline{9}8 & \approx & 300 \end{array}\]

Exercise 3

Round each number to the nearest ten. Check your work using the answer key at the end of the exercise.

1. 424 ≈ _____
2. 867 ≈ _____
3. 499 ≈ _____
4. 132 ≈ _____
5. 278 ≈ _____
6. 617 ≈ _____
7. 208 ≈ _____
8. 851 ≈ _____
9. 124 ≈ _____
10. 576 ≈ _____
11. 315 ≈ _____
12. 742 ≈ _____
13. 397 ≈ _____
14. 952 ≈ _____
15. 639 ≈ _____

Answers to Exercise 3

1. 420
2. 870
3. 500
4. 130
5. 280
6. 620
7. 210
8. 850
9. 120
10. 580
11. 320
12. 740
13. 400
14. 950
15. 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

1. 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

2. 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

3. 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

1. 10, 30, 10
2. 60, 120, 30, 80
3. 680, 490, 390, 910

Topic E Self-Test

Mark /12 Aim 10/12

1. Round your answer to the nearest ten. (8 marks)
   1. 47 ≈ _____
   2. 123 ≈ _____
   3. 4 ≈ _____
   4. 945 ≈ _____
   5. 329 ≈ _____
   6. 481 ≈ _____
   7. 865 ≈ _____
   8. 916 ≈ _____
2. 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 E Self-Test

1. 1. 50
   2. 120
   3. 0
   4. 950
   5. 330
   6. 480
   7. 870
   8. 920
2. 1. 80, 90, 80, 70, 100