Cogito
Grade 1 Math · Chapter 8 · Lesson 2
Subtracting Tens From Tens
Take whole tens away and count what is left.
10 problems · about 10 minutes · 1.NBT.C.6
What this lesson teaches
The child subtracts multiples of ten within 40, including differences of zero.
- Taking away tens leaves whole tens.
- 40 − 10 = 30.
- Taking away all of them leaves 0.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
4 problemsReview — 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.
Review — Greater Than, Less Than, Equal To: What does < mean?
Answer Is less than.
Why < means is less than.
Warm Up
Straightforward practice. Get the method working first.
3 problems30 − 20 = ?
Answer 10
Why One ten is left, which is 10.
40 − 40 = ?
Answer 0
Why Nothing is left, so the answer is 0.
20 − 10 = ?
Answer 10
Why 2 rods take away 1.
Build It Up
The same ideas with more to keep track of.
1 problem40 − 30 = ?
Answer 10
Why 4 rods take away 3.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problems20 − 20 = ?
Answer 0
Why All the rods go.
Sharing Stickers: Ana has 30 stickers. She gives away 10. How many are left?
Answer 20 stickers
Why 30 − 10 = 20.