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

Multiplying by a One-Digit Number

Multiply numbers up to four digits by one digit.

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The area model finds every partial product. The written method below does the same work in a shorter way: one column at a time, carrying as you go.

Key idea

Multiply the ones, then the tens, then the hundreds. When a product is 10 or more, write its ones digit and carry the rest to the next place. In the next place, multiply first, then add the carried amount.

Why it works

In $346 \times 7$, the ones give $6 \times 7 = 42$ ones: 4 tens and 2 ones. The 4 tens join the tens column: $4 \times 7 = 28$ tens, plus the 4 tens makes 32 tens. These are the same partial products as the area model, $2{,}100 + 280 + 42$, added as you go.
Press Next step to multiply one place at a time. The small blue digits are the carried amounts.

Worked example: Carrying twice

Find $346 \times 7$.
  1. Ones: $6 \times 7 = 42$. Write 2, carry 4.
  2. Tens: $4 \times 7 = 28$, plus 4 carried is 32. Write 2, carry 3.
  3. Hundreds: $3 \times 7 = 21$, plus 3 carried is 24. Write 24.
  4. Check with the area model: $2{,}100 + 280 + 42 = 2{,}422$.

Answer: $2{,}422$

Worked example: Four digits

Find $1{,}508 \times 6$.
  1. Ones: $8 \times 6 = 48$. Write 8, carry 4.
  2. Tens: $0 \times 6 = 0$, plus 4 carried is 4. Write 4.
  3. Hundreds: $5 \times 6 = 30$. Write 0, carry 3.
  4. Thousands: $1 \times 6 = 6$, plus 3 carried is 9. Write 9.

Answer: $9{,}048$

Common mistake

Forgetting to add the carried amount, so $346 \times 7$ comes out as $2{,}182$. Another slip is adding the carry before multiplying ($4 + 4 = 8$, then $8 \times 7$). Multiply first, then add the carry. Estimate to check: $350 \times 7 = 2{,}450$, close to $2{,}422$.

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 a whole number of up to four digits by a one-digit whole number)
  • OpenStax Prealgebra 2e, section 1.4 Multiply Whole Numbers
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