Cogito
Grade 4 Math · Chapter 6 · Lesson 5
Adding Tenths and Hundredths
Change the tenths into hundredths, then count.
10 problems · about 15 minutes · 4.NF.C.5
What this lesson teaches
The child adds fractions with denominators 10 and 100 by rewriting the tenths.
- Tenths and hundredths cannot be added as they are.
- Rewrite the tenths as hundredths first.
- Then count the hundredths.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
2 problemsReview — Comparing Decimals: Which is bigger, 0.9 or 0.89?
Answer 0.9.
Why 90 hundredths beats 89 hundredths.
Review — Why 0.7 and 0.70 Are Equal: 0.2 in hundredths. How many?
Answer 20
Why 0.2 = 0.20 = 20/100.
Warm Up
Straightforward practice. Get the method working first.
4 problems7/10 + 15/100. How many hundredths?
Answer 85
Why 85/100, written 0.85.
4/10 written in hundredths. How many?
Answer 40
Why 40/100.
3/10 + 12/100. How many hundredths?
Answer 42
Why 30 + 12.
6/10 + 5/100. How many hundredths?
Answer 65
Why 60 + 5.
Build It Up
The same ideas with more to keep track of.
2 problems1/10 + 45/100. How many hundredths?
Answer 55
Why 10 + 45.
Why must the tenths be rewritten first?
Answer The units have to match before counting.
Why Same rule as adding fractions.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Ribbon: 2/10 metre plus 35/100 metre. How many hundredths in all?
Answer 55 hundredths
Why 55/100, written 0.55.
The Bottle: 5/10 litre plus 12/100 litre. How many hundredths in all?
Answer 62 hundredths
Why 62/100, written 0.62.