Grade 4Multi-Digit Multiplication4.NBT.B.5

Multiplying Two Two-Digit Numbers

Find four partial products and add them.

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Now both factors have two digits. The area model still works: split both numbers by place value, and the rectangle has four parts.

Key idea

For $23 \times 14$, split $23 = 20 + 3$ and $14 = 10 + 4$. Find all four partial products: tens × tens, tens × ones, ones × tens and ones × ones. Then add them.

Why it works

Each part of 23 must be multiplied by each part of 14, because the whole rectangle is $23$ by $14$. Cutting it both ways makes four rectangles. Leave one out and you lose part of the area.
The four parts of $23 \times 14$: $200 + 80 + 30 + 12 = 322$.

Worked example: Four partial products

Find $23 \times 14$.
  1. $20 \times 10 = 200$
  2. $20 \times 4 = 80$
  3. $3 \times 10 = 30$
  4. $3 \times 4 = 12$
  5. Add: $200 + 80 + 30 + 12 = 322$.

Answer: $322$

Worked example: Bigger digits

Find $47 \times 36$.
  1. $40 \times 30 = 1{,}200$ and $40 \times 6 = 240$.
  2. $7 \times 30 = 210$ and $7 \times 6 = 42$.
  3. Add: $1{,}200 + 240 + 210 + 42 = 1{,}692$.
  4. Estimate: about $50 \times 35 = 1{,}750$. Close, so the answer is reasonable.

Answer: $1{,}692$

Common mistake

Multiplying only tens by tens and ones by ones: $2 \times 1 = 2$ and $3 \times 4 = 12$, written as "212". That skips two of the four parts. Check with an estimate: $23 \times 14$ is about $20 \times 15 = 300$, so 212 is too small.

Practice

Pass with 3 right answers, the last 2 in a row. Each right answer earns 2 XP.

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Sources and further reading

  • Common Core State Standards for Mathematics, Grade 4, 4.NBT.B.5 (multiply two two-digit numbers using strategies based on place value)
  • OpenStax Prealgebra 2e, section 1.4 Multiply Whole Numbers
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