Thursday, 7 August 2008

Mental Conversion of Fraction to Decimal - the Quick way

In maths, while doing division of numbers, we will normally come across the fraction, be it improper or proper form. And we may need to have the answer in decimal!

Mentally how do we get the answer fast and simple?

To have this quick conversion from fraction to decimal, we first need to memorise some simple basic fractions. This will speed up the process of mentally computing the conversions.

We can do mental division using any of the method presented in this blog (refer to links at the end of this post). But the answer is in fraction form.

Example is 40 / 7 = 5 5/7, a mixed numer format.

We like to have this 5/7 in decimal (sometimes).

  • Firstly, we need to know (memorise) the below basic fractions/decimal numbers:
    1/2 = 5
    1/3 = 0.333
    1/4 = 0.25
    1/5 = 0.2
    1/6 = 0.1667
    1/7 = 0.1429
    1/8 = 0.125
    1/9 = 0.111
    1/10 = 0.1


You can see that these basic fraction/decimal conversions are not difficult to memorise.
In fact some of them are sub-product of others.

Example: 1/ 6 =(1/2) x (1/3) = 0.5 x 0.333 or 5 x 0.333 / 10
You can choose to memorise 1/2 and 1/3 and do simple mental multiplication later for 1/6.

Another example is 1/8. This can be achieved with (1/2) x (1/4).

After memorising the above basic numbers, we can straight away use them in the computation. Let's see an example.

Example: 42 / 5 ==> 8 2/5

We know that 8 2/5 = 8 + 2/5.

Mentally converting, 8 + 2/5 ==> 8 + (2 x 0.2) = 8.4 (the answer)
This is with the use of the memorised number of 1/5 = 0.2.

Another example: 37 / 8

Mental division produces 37 / 8 = [(8 x 4) / 8] + ( 5 / 8 ) = 4 + ( 5/8 )

We need only to convert the ( 5 / 8 ) into decimal.

5 / 8 = 5 x 1/8 = 5 x 0.125 ==> 0.625 using Mental Multiplication method (from left to right approach).

Therefore 37 / 8 = 4 + 0.625 = 4.625 done mentally!

From the above examples, we can see the effectiveness of the memorised fractions/decimals numbers simplifying the mental conversion process.

Practice with this method and it will get easier with times.

Links for mental division:
Mental Division

Mental Division - denominator approach

Mental Division - Inflation method

Mental Division - Deflation method
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4 comments:

Sinziana Armencea said...

I want to know your opinion on this online converter http://decimal2fraction.com

I like it because it saves me a lot of time (I don't really do well at math) and I think it does the job pretty well.

EeHai said...

Hi Sinziana,

The conversion link pointed in your comment works very well as the shown working is clear. Good site to visit when necessary. It makes maths interesting.

Sinziana Armencea said...

Glad you like it. :)

Julius Alcantara said...

1/2 = 0.5