Rounding and Estimating
Lesson 3-1 and 3-2
Rounding and Estimating
Review of rounding skills:
Identify the place value you are rounding
to.
If the number to the right is 5 or more,
round the identified place value up.
If the number to the right is less than 5,
leave the identified place value as is.
Example 1
Round 4.2683 to the nearest tenth:
“2” is in the tenths place.
The number to the right is “6” which is
greater than 5.
So round the “2” up to “3”.
Answer: 4.3
Example 2
Round 4.2683 to the nearest integer.
The nearest integer is in the ones place--4
The number to the right, “2” is less than 5
The “4” stays as is
Answer: 4
TRY THIS
Round to the underlined place value:
38.41
0.7772
7,098.56
TRY THIS
Round to the underlined place value:
38.41
0.7772
7,098.56
38.4
TRY THIS
Round to the underlined place value:
38.41
38.4
0.7772
1
7,098.56
TRY THIS
Round to the underlined place value:
38.41
38.4
0.7772
1
7,098.56
7,099
Estimation
We can use the skill of rounding to estimate
calculations.
For example, to estimate the sum of
58.4 + 11.1 + 18.9 we could round to
58 + 11 + 19 which equals 88
Or we could round to
60 + 10 + 20 which equals 90
Estimation
Both 88 and 90 are reasonable answers.
Are they EXACT? No, but remember they
are estimates, so both answers are correct.
TRY THIS
Estimate:
355.302 + 204.889
453.56 – 230.07
TRY THIS
Estimate:
355.302 + 204.889
453.56 – 230.07
560
TRY THIS
Estimate:
355.302 + 204.889
560
453.56 – 230.07
220
EXAMPLE
When multiplying numbers we can also
use rounding to estimate.
For example: 7.65 x 3.2
This is about 8 x 3 which equals 24
TRY THIS
Estimate each product:
4.72 x 1.8
17.02 x 3.78
8.25 x 19.8
TRY THIS
Estimate each product:
4.72 x 1.8
17.02 x 3.78
8.25 x 19.8
10
TRY THIS
Estimate each product:
4.72 x 1.8
10
17.02 x 3.78
68
8.25 x 19.8
TRY THIS
Estimate each product:
4.72 x 1.8
10
17.02 x 3.78
68
8.25 x 19.8
160
Example
However when estimating division
problems, it is helpful to use compatible
numbers.
For example when dividing 17.8
4.2
Rather than rounding use the compatible
number of 16 and 4:
16
4 =4
TRY THIS
Find compatible numbers to divide:
38.9 1.79
11.95 2.1
86.52 4.25
TRY THIS
Find compatible numbers to divide:
38.9 1.79
11.95 2.1
86.52 4.25
19 or
20
TRY THIS
Find compatible numbers to divide:
38.9 1.79
19 or
20
11.95 2.1
6
86.52 4.25
TRY THIS
Find compatible numbers to divide:
38.9 1.79
19 or
20
11.95 2.1
6
86.52 4.25
20 or
25