Cogito
Grade 1 Math · Chapter 8 · Lesson 8
Adding Two-Digit Numbers
Add tens to tens, then ones to ones.
10 problems · about 13 minutes · 1.NBT.C.4
What this lesson teaches
The child adds two two-digit numbers within 40 by adding tens to tens and ones to ones, with no new ten needed.
- Add the tens to the tens.
- Add the ones to the ones.
- Then say the answer as tens and ones.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
4 problemsReview — Word Problems Within 40: Whole 35, one part 15. Other part?
Answer 20
Why 35 − 15 = 20.
Review — Tape Diagrams for Stories: Parts of 13 and 12. What is the whole?
Answer 25
Why 13 + 12 = 25.
Review — Making the Next Ten: 29 + 5 = ?
Answer 34
Why 30 and 4 more is 34.
Review — Adding Ones Without Making a New Ten: 31 + 6 = ?
Answer 37
Why 1 one and 6 ones is 7 ones.
Warm Up
Straightforward practice. Get the method working first.
3 problems21 + 14 = ?
Answer 35
Why 3 tens and 5 ones is 35.
What do you add first?
Answer Tens with tens.
Why Like units go together.
11 + 12 = ?
Answer 23
Why 2 tens, then 1 and 2.
Build It Up
The same ideas with more to keep track of.
1 problem14 + 23 = ?
Answer 37
Why 3 tens, then 4 and 3.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problems22 + 16 = ?
Answer 38
Why 3 tens, then 2 and 6.
Two Classes: 12 children in one class and 15 in another. How many?
Answer 27 children
Why 12 + 15 = 27.