WALT: multiply 2/3-digit numbers by a 1-digit number using the column method.
We have been working on the expanded column method in multiplication which is an excellent introduction to the column method. It is often a difficult method to grasp to begin with, so if you are having some problems, please do not worry; keep using the expanded method to check your answers until you are more comfortable.
The multiplications you need to do are the same as the expanded method, it is just reflected slightly differently.
There are 3 challenges in the file in Resources:
Challenge 1: 2-digit numbers multiplied by a 1-digit number with no carrying.
Challenge 2: 2-digit numbers multiplied by a 1-digit number with carrying and 3-digit numbers multiplied by a 1-digit number with no carrying.
Challenge 3: 3 and 4-digit numbers multiplied by 1-digit numbers.
Extension: 2 and 3-digit numbers multiplies by 2-digit numbers.
It may look daunting and difficult to some of you however parts are not easy to explain written down.
I have made a video explaining the method however it is too large to post on here. The link for the video, explaining multiplying 2-digits by 1-digit can be found on this link: https://youtu.be/2fnlze9rRqo
We will first multiply 43 x 2 using this method. This does not require any carrying or exchanging of numbers so is the most straight-forward multiplication sum.
Remember how we always ensure we start by using our place value columns, so we will write the sum like this:
H T O
Step 1: Multiply the numbers in the 'ones' column together, so here we multiply 3 x 2, which equals 6. The answer remains in the 'ones' column, underneath the line.
Step 2: Multiply the number in the 'tens' column by the bottom number, making our sum, 4 x 2. The answer to this sum is '8' and this is placed in the 'tens' column, underneath the line, making the answer '86'.
The sum for Step 2 is actually really 40 x 2 as the '4' is in the tens column however you do not need to do anything with this, other than know the '4' is in the 'tens' column.
We will multiply 76 x 3 using this method. This requires carrying of numbers so is a little more difficult to calculate however as long as you know your 3 times tables, you will be able to work it out.
Once again, remember we always ensure we start by using our place value columns, so we will write the sum like this:
H T O
2 2 8
Step 1: Multiply the numbers in the 'ones' column together, so here we multiply 6 x 3, which equals 18. The 'ones' number in 18, which is '8', goes into the 'ones' column underneath the line. However there is still 1 'ten' that we need to put into our sum. We need to put this in the 'tens' column, above the line, as we will use it a little later in our sum.
Step 2: Multiply the number in the 'tens' column by the bottom number, making our sum, 7 x 3. The answer to this sum is '21' so the '1' would be placed in the 'tens' column, however we have to add the small number '1' that we put above the line. This mean we add 1 + 1, which equals '2' and place this underneath the line in the 'tens' column.
This is why it is imperative that the numbers are lined up properly. This then results in the '2' being placed in the 'hundreds' column, resulting in the answer being '228'.
You can always use the expanded method to check your answer.