Cogito
Grade 1 Math · Chapter 8 · Lesson 3
Adding Tens to Any Number
Add whole tens and watch the ones stay put.
10 problems · about 11 minutes · 1.NBT.C.4
What this lesson teaches
The child adds a multiple of ten to a two-digit number, and explains why the ones digit is unchanged.
- Tens join the tens. Ones stay as they are.
- 12 + 30 = 42.
- Only the tens digit changes.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
4 problemsReview — Subtracting Tens From Tens: 30 − 20 = ?
Answer 10
Why One ten is left, which is 10.
Review — Adding Tens to Tens: 20 + 10 = ?
Answer 30
Why 2 tens and 1 ten is 3 tens.
Review — One More, One Less, Ten More, Ten Less: Ten less than 34 is?
Answer 24
Why 3 tens became 2 tens.
Review — Ten More, Ten Less: What is ten more than 21?
Answer 31
Why 2 tens became 3 tens.
Warm Up
Straightforward practice. Get the method working first.
3 problems16 + 20 = ?
Answer 36
Why 1 ten and 2 tens is 3 tens, with 6 ones.
When you add 10, does the ones digit change?
Answer No. It stays the same.
Why Only the tens digit changes.
15 + 10 = ?
Answer 25
Why The 5 stays.
Build It Up
The same ideas with more to keep track of.
1 problem12 + 20 = ?
Answer 32
Why The 2 stays.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problems17 + 20 = ?
Answer 37
Why 1 ten and 2 tens.
Two Bags: Leo has 13 marbles. He finds a bag of 20. How many?
Answer 33 marbles
Why 13 + 20 = 33.