Subtraction

There are a couple of useful mental methods for subtraction.

Mental Methods

Example:

subtraction-calc

Method 1 – Count On

Count on from 56 until you reach 93.  This can be done in several ways e.g.

subtraction-method1

Method 2 – Break Up the Numbers Being Subtracted

e.g. subtract 50, then subtract 6

subtraction-method2a subtraction-method2b

Written Methods

We use the decomposition method.  The numbers must be written with the units, tens and hundreds correctly in line.

Step 1

Subtract the units. Here, we cannot have 0 – 6 so 10 is added to the zero.  10 – 6 = 4.

subtraction-written1

Step 2

Subtract the tens.  In this case, as 10 was added to the units in the top row, it must now be subtracted from the tens so the 9(0) becomes 8(0).  8 – 8 = 0.

subtraction-written2

Step 3

Subtract the hundreds.  In this case it is simple.  5 – 3 = 2.

subtraction-written3

 

Step 4

Subtract the thousands.

subtraction-written4

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