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**fortunefaded****Member**- Registered: 2005-09-19
- Posts: 1

Hey all,

I'm not very good at maths but help me out, will ya?

Ok, here is the scenario:

The total value of the car is £100, so what is 10%?

ok...thats £10, so new valueis £90, from that...how do you get back to the full amount via percentages.

I would say, ohh its 90% of the total price...so add 10%, right? so this is:

10% of 90 which £9, so the new value is £99, that is incorrect.

How do i do it via percentages. I'm meant to know this for vat purposes, i know in answer it, in the forumla it would be like times 100.2 etc.

please help!

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**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,604

Yes, because 10% of 100 is 10, 10% of 90 is 9, just as 10% of 50 is 5.

When you say "subtract 10%" you are really saying, "take the original number, and subtract 10% of the original number"

So: Answer = Number - Number×10%

Or: Answer = Number - Number×0.10 (because 10% = 0.10)

Or: Answer = Number × (1-0.10)

So: Answer = 100 × (1-0.10) = 100 × 0.9 = 90

You got 90 by multiplying ... so to reverse it you divide: 90 ÷ 0.9 = 100

If you prefer to multiply instead of dividig you can do it this way: 90 × (1/0.9) = 90 × 1.111... = 99.999...

This can be used for any percentage: 120 - 15% = 120 × (1-0.15) = 120 × 0.85 = 102

Reverse it: 102 ÷ 0.85 = 120, or 102 × (1/0.85) = 120 × 1.1765 (approx) = 120.003

For 17.5% the "reduction" is: × (1-0.175) , or × .825

And the "get it back again" is: × 1/(1-0.175) , or × 1.212121...

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..." - Leon M. Lederman

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**Ohlordy!****Guest**

I don't understand where your getting the 1- from.

Also, i thought i had it, it worked when i did you example of 120, but i'll show you my examples:

372-35%

me:

372/100

=3.72*35

=130.0

so:

130.2/65 (should equal 372?)

=200.3077

Second example:

260-75%

me:

260/100

=2.6*75

=195

So

195/0.25

=(should equal 260?)

=780

Third Example:

34-17.5%

=0.34*17.5

=5.95

So

5.95/0.825

=7.2

...::sigh::... I just don't understand what i'm doing wrong

**MathsIsFun****Administrator**- Registered: 2005-01-21
- Posts: 7,604

1 is 100%, so I was just showing that 100%-10% = 90%, or 1-0.1=0.9

Remember you are not just calculating percentages, you are *subtracting* percentages.

372-35% = (372) - (35% of 372) = 372 - 130.2 = 241.8

is the same as:

372 x 65% = 372 x 0.65 = 241.8

To get from 241.8 back to 372: 241.8 / 0.65 = 372

So, be careful of the difference between 10% of 50 (=5) and 50-10% (=50-5=45)

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..." - Leon M. Lederman

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