Cogito
Grade 2 Math · Chapter 5 · Lesson 6
Subtracting Across a Zero
The hardest case, taken slowly.
10 problems · about 15 minutes · 2.NBT.B.7, 2.NBT.B.9
What this lesson teaches
The child subtracts from a number with a zero in the tens place, unbundling twice.
- A zero in the tens means you must break twice.
- Break the hundred first, then one of the new tens.
- The amount never changes, only the grouping.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
3 problemsReview — Breaking a Hundred Apart: 635 − 172 = ?
Answer 463
Why 635 − 172 = 463.
Review — Subtracting Within 1,000: 758 − 326 = ?
Answer 432
Why 758 − 326 = 432.
Review — Making a New Hundred: 275 + 168 = ?
Answer 443
Why 275 + 168 = 443.
Warm Up
Straightforward practice. Get the method working first.
3 problems405 − 137 = ?
Answer 268
Why 405 − 137 = 268.
Why break the hundred before the ten?
Answer There are no tens to break yet.
Why Breaking the hundred is what creates the tens you then break.
502 − 147 = ?
Answer 355
Why No tens to break. Start with a hundred.
Build It Up
The same ideas with more to keep track of.
1 problem600 − 235 = ?
Answer 365
Why Break twice.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
3 problems703 − 158 = ?
Answer 545
Why Only 3 ones and no tens.
The Empty Column: 2 hundreds, 9 tens and 15 ones is which number?
Answer 305
Why It is 305, the same number rearranged.
The Shop Again: A shop had 400 items and sold 156. How many are left?
Answer 244 items
Why 400 − 156 = 244.