Cogito
Grade 2 Math · Chapter 5 · Lesson 7
Adding Up to Four Numbers
Look for pairs that make a friendly number.
10 problems · about 13 minutes · 2.NBT.B.6
What this lesson teaches
The child adds up to four two-digit numbers, reordering them to make the work easier.
- Add several numbers in whichever order is easiest.
- Look for pairs that finish on a ten.
- The total never changes, only the work.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
3 problemsReview — Subtracting Across a Zero: 405 − 137 = ?
Answer 268
Why 405 − 137 = 268.
Review — Breaking a Hundred Apart: 635 − 172 = ?
Answer 463
Why 635 − 172 = 463.
Review — Subtracting Within 1,000: 758 − 326 = ?
Answer 432
Why 758 − 326 = 432.
Warm Up
Straightforward practice. Get the method working first.
3 problems13 + 27 + 20 = ?
Answer 60
Why 40 + 20 = 60.
Does the order you add in change the total?
Answer No. The total stays the same.
Why Only the difficulty changes, never the answer.
15 + 25 + 30 = ?
Answer 70
Why 15 and 25 make 40.
Build It Up
The same ideas with more to keep track of.
1 problem18 + 22 + 14 + 16 = ?
Answer 70
Why 18 and 22 make 40.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
3 problems11 + 29 + 13 + 27 = ?
Answer 80
Why Two pairs, both making 40.
Four Boxes: Boxes hold 12, 28, 15 and 25 apples. How many apples in all?
Answer 80 apples
Why 40 + 40 = 80.
Three Days: Leo read 34, 26 and 18 pages on three days. How many pages in all?
Answer 78 pages
Why 60 + 18 = 78.