Cogito
Grade 4 Math · Chapter 2 · Lesson 5
Estimating Products
Round first, so a wrong answer stands out.
10 problems · about 13 minutes · 4.NBT.B.5, 4.OA.A.3
What this lesson teaches
The child estimates a product by rounding and uses it to judge an exact answer.
- Round both factors, then multiply.
- Compare the exact answer with the estimate.
- It catches answers that are the wrong size entirely.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
3 problemsReview — Two-Digit by Two-Digit: 24 × 13 = ?
Answer 312
Why 24 × 13 = 312.
Review — The Area Model and Partial Products: 42 × 8 = ?
Answer 336
Why 40 × 8 = 320, and 2 × 8 = 16.
Review — Multiplying by 10, 100, and 1,000: 8 × 700 = ?
Answer 5600
Why 8 × 7 = 56, so 8 × 700 = 5,600.
Warm Up
Straightforward practice. Get the method working first.
3 problemsEstimate 38 × 21 to the nearest ten.
Answer 800
Why 40 × 20 = 800.
What kind of mistake does an estimate catch best?
Answer An answer that is wildly the wrong size.
Why It catches misplaced digits, not small slips.
Estimate 19 × 31 to the nearest ten.
Answer 600
Why 20 × 30.
Build It Up
The same ideas with more to keep track of.
1 problemEstimate 58 × 22 to the nearest ten.
Answer 1200
Why 60 × 20.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
3 problemsEstimate 7 × 68 to the nearest ten.
Answer 490
Why 7 × 70.
A Rough Answer: Estimate 41 × 29 by rounding both to the nearest ten.
Answer 1200
Why 40 × 30 = 1,200.
The Exact One: Now work out 41 × 29 exactly.
Answer 1189
Why 1,189, close to the estimate of 1,200.