Cogito
Grade 3 Math · Chapter 2 · Lesson 3
Estimating Sums and Differences
A quick answer that tells you if the real one is sensible.
10 problems · about 12 minutes · 3.NBT.A.1, 3.OA.D.8
What this lesson teaches
The child estimates a sum or difference by rounding, and uses it to judge an exact answer.
- Round each number, then work with the rounded ones.
- An estimate is close, not exact.
- Use it to catch answers that are wildly wrong.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
3 problemsReview — Rounding to the Nearest Hundred: Round 649 to the nearest hundred.
Answer 600
Why 649 is just before halfway, so it rounds to 600.
Review — Rounding to the Nearest Ten: Round 73 to the nearest ten.
Answer 70
Why 73 is before halfway, so it rounds to 70.
Review — Multiplication and Division Word Problems: 5 boxes with 8 pens each. How many pens?
Answer 40
Why 5 × 8 = 40.
Warm Up
Straightforward practice. Get the method working first.
3 problemsEstimate 406 + 195 to the nearest hundred.
Answer 600
Why 400 + 200 = 600.
What is an estimate for?
Answer Checking whether an answer is sensible.
Why An estimate tells you roughly where the answer should be.
Estimate 312 + 289 to the nearest hundred.
Answer 600
Why 300 + 300.
Build It Up
The same ideas with more to keep track of.
1 problemEstimate 587 − 291 to the nearest hundred.
Answer 300
Why 600 − 300.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
3 problemsEstimate 48 + 33 to the nearest ten.
Answer 80
Why 50 + 30.
Two Crowds: 178 and 216 people. Estimate the total to the nearest hundred.
Answer 400 people
Why 200 + 200 = 400.
Checking a Friend: Leo says 412 − 189 = 623. Estimate to check: 400 − 200 is what?
Answer 200
Why The estimate is 200, so 623 must be wrong.