Subtraction Strategies 



Before I show
examples of the different subtraction strategies I am listing some tasks that
it would be useful if children aged 57 were familiar with in
order to achieve their maximum potential.
They will need to:


Scroll down for strategies 

The
strategies for subtraction are based on the same principles as the ones
for addition.
It
is important to remember that we teach many different methods in order
that a child can decide which method works best for them. If it gives you
the right answer, it is the best method for you!!!
Mental Calculations The rule here is that with subtraction you must always start with the largest number and count back. This seems obvious to adults but it does need careful explanation to some children. The best way to explain it is to relate it to an everyday event. For example: using 10  7 explain that if you have 10 sweets you can eat 7 of them but if you have 7 you cannot eat 10. Sounds obvious I know but as adults we sometimes forget these basic principles and assume all children will understand, this however is not always the case. 
"T U
B" Method "T.U.B." stands for Tens, Units, Both. This is a good strategy for subtracting two 2 digit numbers and uses partitioning (splitting numbers into tens and units). Here is an example 25  13 =12 Step 1. Partition the numbers ( 20 + 5 ) ( 10 + 3 ) Step 2. Subtract the tens (put "T" at side) T 20  10 = 10 Step 3. Subtract the units (put "U" at side) U 5  3 = 2 Step 4. Add "T" and "U" answers together (put "B" for both) B 10 + 2 = 12 THE "B" IS AN ADDITION Step 5. Remember to put the answer at the end of the original sum

Empty
Number Line Method
Example 1  subtracting a number below 10 27  4 = 23 Step 1.Draw a horizontal line
with a ruler. 

Step 3.
Put 27 in the circle (represents putting the largest number in your head
as in mental calculation


Step 4.
Draw 4 small jumps from the top of the 'head' and write the number before where each jump touches the line.
Step 5.Remember to write the answer (27) by the original sum. 

Scroll down for 2nd example (subtracting a number 10 or above)  
Scroll down for next method 

100
square method
It is hoped that after learning this method children will be able to visualize the 100 square in their mind and use it both in addition and subtraction. It is an excellent aid to counting in tens because the children can see that the next tens number they need lies under the number they are on. Similarly if they are counting back, the next number they need lies above the one they are on e.g. count on from 12 in tens, the number under 12 is 22 then 32 etc. So they can therefore just follow down the column (or up if subtracting) and eventually they will not need to use the 100 square, they will see it in their mind. 

Example of subtraction using 100 square.  
25  12 =13 Click here for printable 100 square  
Step 1. Partition the 12 into 10 and 2, then locate 25 on the 100 square (shown yellow). Step2. Subtract the 10 (obtained when 12 was partitioned) by going up 1 square to 15 (shown red) Step 3. Count back the 2 units (obtained when 12 was partitioned) by going back 2 squares (shown blue). Answer 13. Step 4. Remember to write the answer by the original sum.To subtract larger 2 digit numbers follow the same method, just move up the required number of tens i.e. for 37, move up 3 tens and back 7 units.


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