Cogito
Grade 5 Math · Chapter 3 · Lesson 2
Subtracting Decimals
Unbundle across the point.
10 problems · about 15 minutes · 5.NBT.B.7
What this lesson teaches
The child subtracts decimals to hundredths, including regrouping across the decimal point.
- Line the places up, filling with zeros.
- A whole trades for 10 tenths, a tenth for 10 hundredths.
- Check by adding the answer back on.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
2 problemsReview — Lining Up the Places: 0.6 + 0.27, in hundredths?
Answer 87
Why 87 hundredths, which is 0.87.
Review — Brackets and Order: (9 + 3) × 4 = ?
Answer 48
Why 48.
Warm Up
Straightforward practice. Get the method working first.
4 problems1 − 0.28, in hundredths?
Answer 72
Why 72 hundredths, which is 0.72.
How do you check a subtraction?
Answer Add the answer back on.
Why The answer plus what you took away should give what you started with.
1 − 0.35, in hundredths?
Answer 65
Why 100 − 35.
0.9 − 0.42, in hundredths?
Answer 48
Why 90 − 42.
Build It Up
The same ideas with more to keep track of.
2 problems2 − 0.75, in hundredths?
Answer 125
Why 200 − 75.
How many hundredths can one tenth be traded for?
Answer 10
Why Each place holds ten of the next.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Change: 5 pounds minus 2.35 pounds. How many hundredths are left?
Answer 265 hundredths
Why 265 hundredths, which is 2.65.
The Race: 10.5 seconds minus 9.86 seconds. How many hundredths is the gap?
Answer 64 hundredths
Why 64 hundredths, which is 0.64 seconds.