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Published byDana Burns Modified over 7 years ago
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.
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