Åk 6–9
 
English/Русский
5.3 Setting up Subtraction
Just like setting up addition (the addition algorithm), the basis for setting up subtraction (the subtraction algorithm) is to subtract hundreds, tens, ones, tenths, hundredths and so forth each for themselves. Because of this, you must place these under/over each other when using the model below. Practice subtracting simple numbers before working with this method.


Example A: 128 – 13

Write the numbers so that the ones and tens are over/under each other.


Subtract the ones:
83 = 5


Subtract the tens: 20 – 10 = 10

Subtract the hundred:100 – 0 = 100

The answer becomes one hundred fifteen.


Example B: 817 – 463

 


Write the number so that the hundreds, ones and tens are above/below each other.




Subtract the ones:
7 3 = 4



Subtract the tens: 10 – 60 = ?
Here you notice that we can decrease 60 from 10.  Instead we carry over a hundred to the tens. Then we get:
(100 + 10) - 60 = 110 – 60 = 50


Subtract the hundreds: Here you see that the eight is crossed out because we have loaned a hundred and there remains seven hundreds.

700 – 400 = 300.

The answer becomes three hundred fifty four.


Example C: 202 – 63 = 139

Set up the numbers so that the thousands, hundreds, ones and tens are written above/below each other!


Subtract the ones. Because we can’t take away 3 from 2, we need to carry over.  Here you notice that the next tens is a 0 and then we must carry over from the hundreds and then the tens. Then we get:
(10 + 2) – 3 = 9
 


Subtract the tens. Think about: 100 = 90
We then get:
100 – 60 = 90 – 60 = 30




Subtract the hundreds.
200 – 0 = 100 – 0 = 100


The answer becomes one hundred thirty nine.


Example D: 24.3 – 3.06 = 21.24

Set up the numbers so that the tens, ones, tenths, and hundredths are written over/under each other.
You know that 24.3 = 24.30.



Subtract the hundreds. Because we can’t take 5 hundredths from 0 hundredths we need to carry over from the tenths.
Then we get 0.10 – 0.06 = 0.04.



Subtract the tenths.
0.3 – 0.0 = 0.2 – 0.0 = 0.2
Write out the decimal.




Subtract the ones.
43 = 1




Subtract the tens.
20 - 0 = 20

The answer becomes twenty one and twenty four hundredths.