Cogito
Grade 4 Math · Chapter 2 · Lesson 4
Two-Digit by Two-Digit
Four pieces instead of two.
10 problems · about 16 minutes · 4.NBT.B.5
What this lesson teaches
The child multiplies two two-digit numbers using an area model and four partial products.
- Split both factors by place value.
- That gives four partial products.
- Add all four to get the answer.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
3 problemsReview — The Area Model and Partial Products: 42 × 8 = ?
Answer 336
Why 40 × 8 = 320, and 2 × 8 = 16.
Review — Multiplying by 10, 100, and 1,000: 8 × 700 = ?
Answer 5600
Why 8 × 7 = 56, so 8 × 700 = 5,600.
Review — Multiplication as Comparison: 4 times as many as 12 is?
Answer 48
Why 4 × 12 = 48.
Warm Up
Straightforward practice. Get the method working first.
3 problems24 × 13 = ?
Answer 312
Why 24 × 13 = 312.
How many partial products does two-digit by two-digit give?
Answer 4
Why Two pieces by two pieces gives four rectangles.
21 × 14 = ?
Answer 294
Why 200 + 80 + 10 + 4.
Build It Up
The same ideas with more to keep track of.
1 problem35 × 12 = ?
Answer 420
Why 300 + 60 + 50 + 10.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
3 problems26 × 23 = ?
Answer 598
Why 400 + 120 + 60 + 18.
The Hall: A hall has 23 rows of 15 seats. How many seats?
Answer 345 seats
Why 23 × 15 = 345.
The Orchard: An orchard has 32 rows of 24 trees. How many trees?
Answer 768 trees
Why 32 × 24 = 768.