Written Multiplication
Multiply two-digit and three-digit numbers by a one-digit number using formal written layout
What a learner can do afterwards
- Set out and solve 47 × 6 using short multiplication
- Set out and solve 234 × 5 using short multiplication with carrying
- Check the answer using estimation (e.g. 234 × 5 ≈ 200 × 5 = 1000)
The lesson
You already know how to set out multiplication in columns. Now you will multiply bigger numbers, where some digits multiply to 10 or more. When that happens, write the ones digit in the answer and carry the tens digit to the next column, just like you do in written addition.
Look at 47 x 6. Multiply the ones first: 7 x 6 = 42. Write the 2 in the ones place and carry the 4 tens above the tens column. Next, multiply the tens: 4 x 6 = 24, then add the carried 4 to get 28. Write 28. The answer is 282.
Now try a three-digit number: 234 x 5. Ones: 4 x 5 = 20, write 0, carry 2. Tens: 3 x 5 = 15, add the carried 2 to get 17, write 7, carry 1. Hundreds: 2 x 5 = 10, add the carried 1 to get 11, write 11. Put it together: 234 x 5 = 1170.
Before you multiply, round the bigger number to check your answer sounds right. For 234 x 5, round 234 to 200: 200 x 5 = 1000. Your exact answer should land near that estimate, not miles away from it.
Carry the extra tens into the next column, and always check your answer with a quick estimate.
Watch it
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.