Warm Up

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Warm Up
Problem of the Day
Lesson Presentation
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Warm Up Estimate. 1. 27 30 2. 85 92 3. 12
28 4. 185 201
900
8,100
300
40,000
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Problem of the Day I am a two-digit decimal
greater than 0.25 and less than . My
hundredths digit is 3 times my tenths digit. What
number am I?
2
5
0.26 or 0.39
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Learn to estimate decimal sums, differences,
products, and quotients.
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Vocabulary
compatible numbers
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Jessie earned 27.00 for baby-sitting. She wants
to use the money to buy a ticket to a water park
for 14.75 and a souvenir T-shirt for 13.20. To
find out if Jessie has enough money to buy both
items, you can use estimation. To estimate the
total cost of the ticket and the T-shirt, round
each price to the nearest dollar, or integer.
Then add the rounded values.
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14.75
7
7 gt 5, so round to 15
15
13
13.20
2
2 lt 5, so round to 13

28
The estimated cost is 28, so Jessie does not
have enough money to buy both items.
To estimate decimal sums and differences, round
each decimal to the nearest integer and then add
or subtract.
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Additional Example 1A Estimating Sums and
Differences of Decimals
Estimate by rounding to the nearest integer.
A. 4.5 8.9
5
5 5, so round to 5.
9 gt 5, so round to 9.
Estimate
14
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Additional Example 1B Estimating Sums and
Differences of Decimals
Estimate by rounding to the nearest integer.
B. 28.3 11.7
28
3 lt 5, so round to 28.
7 gt 5, so round to 12.
Estimate
16
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Additional Example 1C Estimating Sums and
Differences of Decimals
Estimate by rounding to the nearest integer.
C. 57.2 (23.72)
57
2 lt 5, so round to 57.
7 gt 5, so round to 24.
33
Estimate
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Try This Example 1A
Estimate by rounding to the nearest integer.
A. 6.5 4.9
7
5 5, so round to 7.
9 gt 5, so round to 5.
Estimate
12
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Try This Example 1B
Estimate by rounding to the nearest integer.
B. 19.2 13.6
19
2 lt 5, so round to 19.
6 gt 5, so round to 14.
Estimate
5
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Try This Example 1C
Estimate by rounding to the nearest integer.
C. 61.3 (18.84)
61
3 lt 5, so round to 61.
8 gt 5, so round to 19.
42
Estimate
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You can use compatible numbers when estimating.
Compatible numbers are numbers that replace the
numbers in the problem and are easier to use.
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Additional Example 2A Estimating with Decimals
Use compatible numbers to estimate.
A. 45.99 2.31
Round to the nearest multiple of 10.
50
3 lt 5, so round to 2.
100
Estimate
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Additional Example 2B Estimating with Decimals
Use compatible numbers to estimate.
B. 51.33 (7.98)
51 is prime, so round to 49.
51.33
49
7.98
7 divides into 49 without a remainder.
7
49 7 7
Estimate
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Try This Example 2A
Use compatible numbers to estimate.
A. 39.62 4.99
Round to the nearest multiple of 10.
40
9 gt 5, so round to 5.
200
Estimate
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Try This Example 2B
Use compatible numbers to estimate.
B. 19.42 (4.88)
19 is prime, so round to 20.
19.42
20
4.88
5 divides into 20 without a remainder.
5
20 5 4
Estimate
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Additional Example 3 Application
Cara spent 58.80 on 4.8 pounds of lobster
and calculated that she spent about 12 per pound
of lobster. Use estimation to check whether her
calculation is reasonable.
60
58.80
Round to the nearest multiple of 10.
8 gt 5, so round to 5.
5
4.85
Estimate
60 5 12
The estimate is the same as Caras calculation.
The lobster cost about 12 per pound.
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Try This Example 3
Clem spent 18.43 on 11.7 pounds of apples. Is it
reasonable to say that Clem spent about 2 per
pound on apples?
Round to the nearest multiple of 10.
20
18.43
10 divides evenly into 20 without a remainder.
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11.7
Estimate
20 10 2
The estimate is the same as Clems calculation.
The apples cost about 2 per pound.
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Lesson Quiz Part 1
Estimate by rounding to nearest integer. 1. 68.4
29.5 2. 7.24 (10.5) Use compatible
numbers to estimate. 3. 48.27 ? 4.67 4. 54.37 ?
5.67
98
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Possible answers
250
9
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Lesson Quiz Part 2
Use compatible numbers to estimate. 5. Ray is
catering a party for 60 people. He needs to have
30 pounds of potato salad. He has made 6.21
pounds and 22.68 pounds. Does he have enough for
the party?
no