Cogito
Grade 5 Math · Chapter 3 · Lesson 3
Estimating with Decimals
Round to the nearest whole one first.
10 problems · about 14 minutes · 5.NBT.B.7, 5.NBT.A.4
What this lesson teaches
The child estimates decimal sums and differences and uses the estimate to judge an answer.
- Round both numbers to the nearest whole one.
- Compare the exact answer with the estimate.
- It catches a decimal point in the wrong place.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
2 problemsReview — Subtracting Decimals: 1 − 0.28, in hundredths?
Answer 72
Why 72 hundredths, which is 0.72.
Review — Lining Up the Places: 0.6 + 0.27, in hundredths?
Answer 87
Why 87 hundredths, which is 0.87.
Warm Up
Straightforward practice. Get the method working first.
4 problemsEstimate 7.8 + 2.1 to the nearest whole numbers.
Answer 10
Why About 10.
An answer of 0.8 to 4.5 + 3.2. Is it sensible?
Answer No. It should be near 8.
Why The answer is ten times too small.
Estimate 3.7 + 5.2 to the nearest whole numbers.
Answer 9
Why 4 + 5.
Estimate 12.1 − 4.9 to the nearest whole numbers.
Answer 7
Why 12 − 5.
Build It Up
The same ideas with more to keep track of.
2 problemsEstimate 8.45 + 2.6 to the nearest whole numbers.
Answer 11
Why 8 + 3.
What kind of mistake does an estimate catch best?
Answer A decimal point in the wrong place.
Why Estimates are rough on purpose.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsA Rough Answer: Estimate 6.9 + 4.2 by rounding each to the nearest whole number.
Answer 11
Why About 11.
The Difference: Estimate 9.8 − 3.1 by rounding each to the nearest whole number.
Answer 7
Why About 7.