Bài 17: Multiplying a three digit by a one digit number

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Carrying over, Starry do you remember, 
what carrying over is?

We learn about it when multiply two digit numbers.

Oh, you have forgotten?
Don’t worry, we will revise it.

Let see we multiply 12 by 7.

We begin by arranging the multiplication vertically.

Next we multiply the units digit 2 in 12 by 7.

The product is 14, we write the 4 of 14 in the units place of the product.

And carry over the 1 ten from 14 to the tens column and write it above the ten in the multiplicand that is above 1.

Now multiply the tens digit 1 in 12 by 7, we get 7 tens, add to this the carried over 1, we get 8 tens.

Write it in the product in the ten places.

The product is 84.

Starry, that was multiply a two digit number by a one digit number by carrying over to the tens place.

Now let us see how to multiply a three digit number by a one digit number again by carrying over to the tens place.

Multiply 224 by 3.

First arrange multiplication vertically, 
next we multiply the units digit 4 in 224 by 3.
The product is 12.

The number 12 has 2 in the units place and 1 in the tens place.

Write the 2 of 12 in the units place and carry over the 1 tens from 12 to the tens column.

And write it above the tens in the multiplicand that is above 2.

Next multiply the tens digit 2 in 224 by 3, we get 6 tens.

Add to this the carried over 1, we get 7 tens.

Write it in the product in the tens place.

Multiply the hundreds digit 2 in 224 by 3, we get 6 hundreds.

Write it in product in the hundreds place.
The product is 672.

Oh, wasn’t that long Starry?
Yes, but very easy.

Now will you help me do another one Starry?

Let us multiply 318 by 2.

First multiply the units digit 8 in 318 by 2, the product is 16.

Write the 6 of 16 in the units place and carry over the 1 tens in 16 to the tens column, write the 1 tens above the tens in the multiplicand, that is above 1.

Next, multiply the tens digit 1 in 318 by 2, we get 2 tens.

Add to this the carried over 1, we get 3 tens.

Write it in the product in the tens place.

Multiply the hundreds digit 3 in 318 by 2, we get 6 hundreds.

Write it in the product in the hundreds place.
The product is 636.

Now let us revise what we have done.

Multiply 119 by 3.

First multiply the units digit 9 in 119 by 3, the product is 27,

write the 7 of 27 in the units place and carry over 2 tens from 27 to the tens column.

Write the 2 tens above the tens in the multiplicand, that is above 1.

Next, multiply the tens digit 1 in 119 by 3, we get 3 tens.

Add to this the carried over 2, we get 5 tens.

Write it in the product in the tens place.

Multiply the hundreds digit 1 in 119 by 3, we get 3 hundreds.

Write it in the product in the hundreds place.
The product is 357.

Now one last multiplication, 
multiply 109 by 9.

First multiply the units digit 9 in 109 by 9. The product is 81.

Write the 1 of 81 in the units place, and carry over the 8 tens from 81 to the tens column.

Write the 8 tens above the tens in the multiplicand that is above 0.

Next, multiply the ten digit 0 in 109 by 9, we get 0 ten.

Add to this the carried over 8, we get 8 tens.

Write it in the product in the tens place.

Multiply the hundreds digit 1 in 109 by 9 , we get 9 hundreds.

Write it in the product in the hundreds place.
The product is 981.

Starry, Wasn’t that a good lesson on multiplying a three digit number by a one digit number.

I sure you enjoyed it.

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