Cogito
Grade 5 Math · Chapter 4 · Lesson 2
Multiplying a Decimal by a Whole Number
Count in tenths, then say what you counted.
10 problems · about 15 minutes · 5.NBT.B.7
What this lesson teaches
The child multiplies a decimal by a whole number by working in the smaller unit.
- Read the decimal as a count of tenths or hundredths.
- Multiply the count as a whole number.
- Then say what that many units comes to.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
2 problemsReview — Multiplying by Powers of Ten: 0.08 × 100 = ?
Answer 8
Why 8.
Review — Estimating with Decimals: Estimate 7.8 + 2.1 to the nearest whole numbers.
Answer 10
Why About 10.
Warm Up
Straightforward practice. Get the method working first.
4 problems9 × 0.3, in tenths?
Answer 27
Why 27 tenths, which is 2.7.
Why work in tenths instead of decimals?
Answer It turns the question into whole numbers.
Why Counting units makes it a whole-number multiplication.
3 × 0.7, in tenths?
Answer 21
Why 3 × 7 tenths.
8 × 0.2, in tenths?
Answer 16
Why 8 × 2 tenths.
Build It Up
The same ideas with more to keep track of.
2 problems5 × 0.12, in hundredths?
Answer 60
Why 5 × 12 hundredths.
7 × 0.05, in hundredths?
Answer 35
Why 7 × 5 hundredths.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Ribbons: 5 ribbons of 0.6 metres each. How many tenths in all?
Answer 30 tenths
Why 30 tenths, which is 3 metres.
The Cost: 4 items at 0.25 pounds each. How many hundredths in all?
Answer 100 hundredths
Why 100 hundredths, which is exactly 1 pound.