AB: required BC: required
MB: required ON: recommended
Teacher Resource 6.1 Unit 2 Number Sense pp. C-29–38
New Canadian Edition
JUMP Math™ Copyright © 2018 JUMP Math
NS6-6
Estimating in Addition and Subtraction
Students will:
• estimate sums and differences and choose a method of estimating.
AP Book 6.1 pp. 35–37
2 | 4 | 7 | 5 | 8 |
| | | | |
Let's round 24 758 to different places.
nearest ten thousand:
2 | 4 | 7 | 5 | 8 |
| | | | |
nearest thousand:
2 | 4 | 7 | 5 | 8 |
| | | | |
nearest hundred:
See p. C-30 for details.
475 + 321 ≈ 500 + 300 = 800
"approximately equal"
Exercises:
Estimate by rounding each number to the stated place value.
a) 421 + 159, tens
See p. C-31 for details.
The first place value in a number is called the leading digit.
Estimating to the leading digit is a good strategy, but it doesn't always work. Sometimes you need to estimate to a different digit.
Exercises:
Estimate the answer by rounding to the leading digit. Then calculate the exact answer.
a) 143 − 127
b) 789 + 567
Discuss the results students got. See p. C-31 for details.
See p. C-32 for details.
48 329 ≈ 48 300
Which digit did I round to?
How do you know?
Exercises:
What digit was the number rounded to?
a) 39 000
b) 4320
If a number has many zeros, it doesn't necessarily mean it's been rounded.
See examples on the next slide.
See p. C-32 for details.
Addition | 4567 + 2731 |
Regular rounding | |
Round one up and another down | |
Actual answer | |
Closer method | |
There are other ways to estimate an addition besides rounding to the leading digit.
Exercises:
Use two methods of rounding. Then calculate �the actual answer.
Which method produces a closer answer?
a) 7234 + 2091
See p. C-33 for details.
It makes sense to use "one up, one down" estimation when both numbers are in the middle of the range.
Hint: The second digits will be 3, 4, 5, 6, or 7.
Then the "mistakes" will cancel each other.
See pp. C-33–34 for details.
Example: Estimate 442 + 543 + 802.
To estimate a sum of three or more numbers, �you might want to use a combination of methods.
See p. C-34 for details.
Subtraction | 4567 − 2431 |
Regular rounding | |
Round both numbers up | |
Actual answer | |
Closer method | |
Now we will round both numbers up to estimate differences.
273 + 385
See p. C-34 for details.
Don added these two numbers and got 958.
Does that answer seem reasonable?