Cogito
Grade 5 Math · Chapter 3 · Lesson 1
Lining Up the Places
Tenths go with tenths.
10 problems · about 15 minutes · 5.NBT.B.7
What this lesson teaches
The child adds decimals to hundredths and explains why the places must line up.
- Tenths add to tenths, hundredths to hundredths.
- Fill short decimals with zeros so the places match.
- A trailing zero never changes the value.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
2 problemsReview — Brackets and Order: (9 + 3) × 4 = ?
Answer 48
Why 48.
Review — A Fraction Is a Division: 5 ÷ 8 as a fraction. What is the top number?
Answer 5
Why 5/8.
Warm Up
Straightforward practice. Get the method working first.
4 problems0.6 + 0.27, in hundredths?
Answer 87
Why 87 hundredths, which is 0.87.
Why must the places line up?
Answer Only matching units can be added.
Why Tenths add to tenths, hundredths to hundredths.
0.4 + 0.25, in hundredths?
Answer 65
Why 40 + 25.
0.7 + 0.08, in hundredths?
Answer 78
Why 70 + 8.
Build It Up
The same ideas with more to keep track of.
2 problems0.35 + 0.65, in hundredths?
Answer 100
Why That makes a whole one.
Does writing 3.4 as 3.40 change its value?
Answer No.
Why A trailing zero adds nothing.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Shopping: 2.50 pounds plus 1.35 pounds. How many hundredths in all?
Answer 385 hundredths
Why 385 hundredths, which is 3.85.
The Ribbon: 1.6 metres plus 0.45 metres. How many hundredths in all?
Answer 205 hundredths
Why 205 hundredths, which is 2.05.