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We can multiply a two-digit number or a three-digit number by a one-digit number using the formal (efficient) method
Year 4 Unit 13b
What we are learning:
Once children understand how to multiply larger numbers it is helpful to find quick written methods
This is an example of how to set out the formal written method:
We always start from the units column, so 2×2 equals 4 and 2 x 40 equals 80 (or 2 x 4 tens = 8 tens). It is really important to put each part of the answer in the correct column that shows its real value.
In the following example we have more than 10 units so we have to exchange them for tens and carry them into the tens column.
We say 6 multiplied by 4 is 24, which is 2 tens carried underneath into the tens column and 4 units which are entered into the units column.
Then we multiply 6 by 10 which is 60, add the 2 tens we carried which gives 8 tens or 80.
In this example we need to carry numbers forward into the tens and hundreds column:
As before we start in the units column, 4 times 3 equals 12. We cannot put the two digits of 12 in one column so we exchange 10 units for one ten which we carry underneath in the tens column, and leave the 2 in the units column.
Then we multiply the 3 by the 5 tens which gives 15 tens (or 150) and add on the 10 we have carried, making 16 tens or 160. We place the 6 of the tens in the tens column and exchange the other 10 tens for one hundreds which we
carry below the line to add on in the next stage
Finally we multiply the 3 by the 100 which gives 300, then we need to add on the 100 we have carried which gives us 400.
Activities you can do at home:
Work through the following practice multiplication questions together.
Good questions to ask and discuss:
When your child is working through a calculation like this that has a number of stages, ask them to ‘think out loud’, that is to tell you what they are doing at each stage and explain it to you. If they go wrong, just ask, ‘Are you sure?’
to see if they can correct themselves.
If your child:
Loses track of the sequence of activity part way through the calculation
Ask them to talk you through what they have done so far and then ask ‘What do you think we have to do next?’