Cogito
Grade 2 Math · Chapter 3 · Lesson 10
A Hundred More, A Hundred Less
The same trick, one place further left.
10 problems · about 12 minutes · 2.NBT.B.8
What this lesson teaches
The child mentally adds and subtracts 100 from any number between 100 and 900.
- A hundred more changes only the hundreds digit.
- Ten more changes only the tens digit.
- Knowing which digit to move is the whole trick.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
2 problemsReview — Ten More, Ten Less: Ten more than 236 is?
Answer 246
Why 236 + 10 = 246.
Review — Using Greater Than and Less Than: Which symbol makes this true: 415 ? 451
Answer <
Why 415 is less than 451.
Warm Up
Straightforward practice. Get the method working first.
4 problemsA hundred more than 546 is?
Answer 646
Why 546 + 100 = 646.
Which digit changes when you add 100?
Answer The hundreds digit.
Why Adding a hundred adds one flat, so the hundreds digit moves.
A hundred more than 415 is?
Answer 515
Why The 1 and 5 stay.
A hundred less than 862 is?
Answer 762
Why One flat fewer.
Build It Up
The same ideas with more to keep track of.
2 problemsTen more than 380 is?
Answer 390
Why This one is tens, not hundreds.
A hundred more than 209 is?
Answer 309
Why The 0 and 9 stay.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Fundraiser: A school has raised 254 pounds and gets 100 more. How much now?
Answer 354 pounds
Why 254 + 100 = 354.
Spending: A shop had 738 apples and sold 100. How many are left?
Answer 638 apples
Why 738 − 100 = 638.