Cogito
Grade 2 Math · Chapter 4 · Lesson 5
Subtracting Tens and Ones
Take the tens away, then the ones.
10 problems · about 11 minutes · 2.NBT.B.5
What this lesson teaches
The child subtracts a two-digit number when every digit is large enough, with no unbundling.
- Take tens from tens and ones from ones.
- Check the ones before you start.
- This works while there are enough of each.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
2 problemsReview — Explaining Why It Works: Someone says the bundled ten is "a one". Are they right?
Answer No. It is one ten.
Why It is one ten, which is why it joins the tens column.
Review — Adding Past 100: 58 + 67 = ?
Answer 125
Why 58 + 67 = 125.
Warm Up
Straightforward practice. Get the method working first.
4 problems95 − 32 = ?
Answer 63
Why 95 − 32 = 63.
What do you check before you start subtracting?
Answer Whether there are enough ones.
Why Checking the ones tells you whether you will need to break a ten.
57 − 23 = ?
Answer 34
Why 5 tens take 2 tens.
89 − 45 = ?
Answer 44
Why 8 tens take 4 tens.
Build It Up
The same ideas with more to keep track of.
2 problems76 − 40 = ?
Answer 36
Why The ones do not change.
68 − 8 = ?
Answer 60
Why Only the ones change.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Jar: A jar had 79 sweets. 34 were eaten. How many are left?
Answer 45 sweets
Why 79 − 34 = 45.
The Reading Log: Leo planned to read 96 pages and has read 42. How many are left?
Answer 54 pages
Why 96 − 42 = 54.