lesson 2 digits multiplication

Untitled Lesson

By Jose Salazar

Standard Algorithm - 2 by 2 digit

Challenge: I can FLUENTLY multiply 2 by 2 digit Numbers by following each step of standard algorithm.

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Fill in the Blanks

39 x 41 = ?

Multiple Choice

Which number goes in the box to make the problem true?

• 2

• 4

• 6

• 8

Multiple Choice

Which numbers do I multiply first?

• 6 x 2

• 6 x 8

Multiple Choice

Which numbers do I multiply next?

• 6 x 2

• 6 x 8

Multiple Choice

Why do we need to add a zero to the second line?

• I like the way it looks.

• It shows that we are starting to multiply in the tens place.

• You can add a zero anywhere you want.

Multiple Choice

What numbers do I multiply next?

• 3 x 2

• 3 x 8

Now that we have the numbers lined up, it’s time to start multiplying.

We start with multiplying the ones place column of the lower number: 3 into 372.

Now we start with the ones places in each of the 2- and 3-digit numbers. In our example that is 3 x 2. 3 x 2 = 6.

Next, we multiply the 3 to the tens number in the top number. In our example, it’s 3 x 7. 3 x 7 = 21. Now this a two-digit number, so we need to pay special attention to where the numbers go in the columns. The ten number in our sum: the 2 will need to be carried. Some people put that number on the side, but a better place for the carried number is on top of the hundreds column. That way, you won’t forget it.

In our next equation, we won’t forget that the 2 needs to be added to the sum.

Now, we multiply the hundreds number in the top number by the ones number below. In our example, that is: 3 x 3. 3 x 3 = 9. We also need to add the carried 2. 9 + 2 = 11. As this is the last number on this line, we put 11 under the line.

We also usually put a line through the carried number once used, so as not to confuse ourselves when we work out the tens number multiplications.

Phew, we’re halfway there.

Now, we do the same for the tens. First, let’s add a zero down the ones column before we move onto the tens column. This will help us not get confused over where the numbers for the tens multiplications start on this new line.

Multiply the bottom tens place number with the ones place number in the top number. In our example, that’s 4 x 2. 4 x 2 = 8.

Next is the tens places turn. We multiply 4 x 7 in our example. 4 x 7 = 28. Again, we have a number that needs to be carried – the 2

Finally, we need to multiple the tens value number on the bottom number with the hundreds place number in the top number. That is 4 x 3. 4 x 3 = 12. Let’s not forget the carried number: 12 + 2 = 14.

From here on, we add the two lines of sums: