Cogito
Grade 2 Math · Chapter 4 · Lesson 4
Explaining Why It Works
Say what you did and why it was allowed.
10 problems · about 12 minutes · 2.NBT.B.9
What this lesson teaches
The child explains an addition strategy in terms of place value rather than reciting steps.
- You may only add units of the same size.
- Bundling is allowed because the amount does not change.
- Explaining why is how you catch your own mistakes.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
3 problemsReview — Adding Past 100: 58 + 67 = ?
Answer 125
Why 58 + 67 = 125.
Review — Making a New Ten: 29 + 34 = ?
Answer 63
Why 5 tens and 13 ones becomes 6 tens and 3 ones.
Review — Adding With Place Value: 23 + 41 = ?
Answer 64
Why 23 + 41 = 64.
Warm Up
Straightforward practice. Get the method working first.
3 problemsSomeone 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.
56 + 38 = ?
Answer 94
Why 56 + 38 = 94.
Why do you add the tens to the tens?
Answer Because they are the same size unit.
Why Think about apples and apples.
Build It Up
The same ideas with more to keep track of.
1 problem48 + 27 = ?
Answer 75
Why 8 and 7 is 15.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
3 problemsIn 48 + 27, how many loose ones are there before bundling?
Answer 15
Why 8 and 7.
Spot the Mistake: Someone works out 34 + 25 and gets 95. What went wrong?
Answer They added a ten to a one.
Why Units were mixed. Tens must go with tens.
Why Bundle?: Why is it allowed to swap ten ones for one ten?
Answer They are worth the same amount.
Why A rod is ten cubes, so nothing changes when you swap them.