## Sunday, 27 July 2008

### Mental Subtraction - The Fast Way

Mental Subtraction is similar to Mental Addition with a bit of deviation.

The steps to compute the answer to the subtraction is still the unconventional left-to-right approach. This allows the answer to the mental subtraction to be recalled straight from the left digit.

We can do an example using the left-to-right approach.

5 4 3
3 2 2 -
------

We start from left digit by doing 5 - 3 = 2, next, the centre digit 4 - 2 = 2,
and finallyright digit 3 - 2 = 1==> Answer is 221.

You can get this with the conventional right-to-left approach but need to flip the final answer to get 221.

How about this subtraction which is a bit more challenging?

4 0 0
3 1 6 -
-------

This calls for a bit of deviation and creativity.

Mental maths operation lies in simplification to achieve speed. In the above last question, we can actually simplify the 316 by splitting it to 300 + 10 + 6.

This makes the mental subtraction easier in that the first subtraction deals with a simplier number of 300.

Later the answer is followed by subtracting 10 and lastly subtracting 6.

400 - 300 = 100
100 - 10 = 90
90 - 6 = 84
That is the final answer !

See how amazingly simple mental subtraction can be when simplified.

Another example of mental subtraction (with deviation)
4 0 0
2 9 4 -
------

For this question, we note that 294 is very close to 300.

So we mentally subtract using the 300 ==> 400 - 300 = 100

But since we over-subtract by 6, we return 6 to the above subtracted answer
===> 100 + 6 = 106.
That it, the final answer done mentally!

With practice using some creativity, mental maths can be fun and fast, and impressive too.
See another method of simplification for mental maths at this link.

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#### 3 comments:

Anonymous said...

Thanks.. Hihi I've never thought this simple thing.. Haha Once again.. You are the best ^^

Anonymous said...

u made mentally working on subtraction so easy...Thanx a lot frnd....

Anonymous said...

Yeah. Good. I'm from Sri Lanka.