Transcript Basic Column Subtraction and Decomposition

Basic Column
Subtraction
1. Write out the numbers that are being subtracted
below each other. When subtracting the largest
number is always written at the top and the units,
tens, hundreds etc. must always be in the same
column.
2. Firstly, start at the units column (far right).
9 – 2 is 5 so that is written underneath.
3. Finally, subtract the numbers in the tens column
(next along to the left) and write the number below.
We encourage the children to think of this as
subtracting e.g. 5 tens from 8 tens, to reinforce that
they are tens not units. If you are solving a sum with
more than 2 digits, keep moving along the columns
from right to left until all columns have been
subtracted.
Column
Subtraction
with
Decomposition
1. Firstly, as with basic column subtraction write the
numbers on top of each other with largest number
at the top.
Remember to ensure that the
hundreds, tens and units are in the correct
column.
2. We start by subtracting the units column first (far right).
When subtracting we always start with the largest number
but here the larger number is at the bottom. The sum would
be 4 - 6 which we can’t do. So we have to exchange (borrow)
from the tens column to make the unit number bigger. We
exchange 1 ten from the tens column so 5 tens becomes 4
tens (we cross out the 5 and write 4 above it). We then give
this 1 ten to the units column making the 4 units into 14 units.
This is because 1 ten and 4 units is the same as 14 units. Now
that the top unit number is bigger we can do the sum 14 – 6 =
8.
3. We can now move onto the tens column. Because we
exchanged 1 ten the sum is now 4 tens – 2 tens
which we can do because the top number is the
biggest. 4 tens – 2 tens = 2 tens (this is written
below).
4. Finally in the hundreds column we are able to
complete the sum because the largest number is on
the top. 7 hundreds – 2 hundreds = 5 hundreds which
is then written underneath.
You only need to
exchange if the top number is smaller than the
number below it and this can be done in any column.
You just need to remember that we always exchange
from the column to the left because the numbers in
that column have a larger value.