Percentages?

[mrs_nordling]

I couldn't think where to post this question. How do I calculate the percentage rate that's been added to an item so that when I re-add it it's correct? I know that seems to make no sense, so here's an example: Sum of 100 has had 10% already added to it's original total, if I deduct 10% I'll have 90 but then adding 10% on to that figure only gives me 99.9. Are you following me so far? So what's the mathematical equation I need to use to figure out the exact percentage rate to deduct?

[roslyn251254]

Eg - if 10% has been added , to get back to the original, you have to divide by 110 then multiply by 100

[factor-fiction]

And in your example when you added 10% to 90 you should have got 99 not 99.9

[R1Geezer]

This is most often applied in VAT. This is one of many ways. Supposing N is the amount with vat in it, it is the result of multiplying the principle amount by 1.175 so to arrive back at the principle simply divide N by 1.175

Similarly to calculate a percentage that has been added you need to subtract the smaller from the larger and then divide the larger into the smaller and multiply by 100. Eg so if we started with 80 and we added X% and finished with 100 then:
100-80 = 20
20/80 = 0.25
x 100 = 25%

[R1Geezer]

so if 100 has had 10% added then the principle amount was 100/1.1=90.909090...

[mrs_nordling]

Thanks for the formulae R1Geezer and yes, you're right factor30, my mistake, of course it's only 90. mrs_n