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#1 2005-09-20 21:45:20
- fortunefaded
- Novice
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Percentages...so basic :(
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!
#2 2005-09-20 22:41:18
- MathsIsFun
- Administrator
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Re: Percentages...so basic :(
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
#3 2005-09-21 09:32:46
- Ohlordy!
- Guest
Re: Percentages...so basic :(
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 
#4 2005-09-21 23:10:37
- MathsIsFun
- Administrator
Offline
Re: Percentages...so basic :(
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