Cogito
Grade 2 Math · Chapter 4 · Lesson 6
Breaking a Ten Apart
Not enough ones? Open a ten.
10 problems · about 13 minutes · 2.NBT.B.5, 2.NBT.B.9
What this lesson teaches
The child unbundles one ten into ten ones and recognizes that the amount is unchanged.
- If there are not enough ones, break a ten open.
- One ten becomes ten ones.
- The amount stays exactly the same.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
3 problemsReview — Subtracting Tens and Ones: 95 − 32 = ?
Answer 63
Why 95 − 32 = 63.
Review — 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.
3 problems6 tens and 12 ones is which number?
Answer 72
Why 6 tens and 12 ones is 72.
When do you need to break a ten open?
Answer When there are not enough ones.
Why Only when the ones you have are too few.
3 tens and 14 ones is which number?
Answer 44
Why 30 and 14.
Build It Up
The same ideas with more to keep track of.
1 problem54 with a ten broken open has how many ones?
Answer 14
Why 4 ones plus ten more.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
3 problemsDoes breaking a ten change the amount?
Answer No. It is only rearranged.
Why Count the cubes before and after.
Another Way to Show It: 4 tens and 13 ones is which number?
Answer 53
Why 4 tens and 13 ones is 53.
Opening One Up: 85 has a ten broken open. How many ones are there now?
Answer 15
Why 7 tens and 15 ones is still 85.