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I wonder if you can help me? I have found similar answers to this question but they don't seem to work.

I am after a formula or an excel formula so I can add a percentage to a number, then when I discount the same percentage off the larger figure I get back to exactly the same figure.

The reason for this is because I sell products at a price, some of my customers have special terms when they purchase so I add a percentage onto the selling price to cover these terms. Then when this percentage is discounted off the inflated cost, I need the selling total to go back to the original selling price.

For instance: If I sell at £1.00, Customer A has terms of 10%, so I need to add 10% onto 1.00, which is £1.10. Then when I take 10% off £1.10, which is £0.11p, this returns to £0.99p, which is lower than my original selling price.

If anyone could help with a formula to explain the answer it would be amazing! I would need the formula to work with different selling prices and different percentages.

Thanks in advance! Adam

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You will need to modify the percentages. If you add $x\%$ to a price $P$ then your new price is $Q=P+\frac{Px}{100}$. You now want to subtract $\frac{Px}{100}$ from $Q$, but that is $x\%$ of $P$, not $x\%$ of $Q$. As a proportion of $Q$, it is $$\frac{\frac{x}{100}}{1+\frac{x}{100}}=\frac{x}{100+x},$$ so this means that instead of subtracting $x\%$ you need to subtract $y\%$ of $Q$, where $$y=\frac{100x}{100+x}.$$ For example, if you added $10\%$ you must subtract $\frac{1000}{110}\%=9.\overline{09}\%$.

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  • $\begingroup$ Thank you so much for your time commenting on my post! I'm still a bit confused, maths isn't my strong point I'm afraid. How would this work If I used as selling price of £1.56, added on 16.5% and then wanted to subtract 16.5% from £1.18174 to get back to £1.56? How would that work in that formula you supplied? Thanks in advance! $\endgroup$
    – AdamF
    Jul 26 '18 at 10:43
  • $\begingroup$ No problem :) You need to subtract $\frac{1650}{116.5}\%$, which is about $14.164\%$. $\endgroup$ Jul 26 '18 at 10:52
  • $\begingroup$ eeeek. So £1.56 * 1.165 = £1.8174p, which is 16.5% increase of £1.56. Then to deduct 16.5% from £1.8174 I need to subtract 14.164% from £1.8174? Doesn't that leave me with £1.5919203p ? I'm so confused haha $\endgroup$
    – AdamF
    Jul 26 '18 at 11:12
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If you take $99\%$ off $€100$, you get $€1$. Then if you add $99\%$ to $€1$, you get $€1.99$, much less than $€100$ because you didn't consider $99\%$ of the same thing.

In general adding $x\%$ is multiplying by $1+\frac x{100}$, so if you want the reverse of that you need to divide by $1+\frac x{100}$, which is the same as multiplying by $\frac{1}{1+\frac x{100}}$. Now

$$\frac{1}{1+\frac x{100}}=1-\frac{x}{100+x}$$ so the reverse of adding $x\%$ is subtracting $(100\cdot \frac{x}{100+x})\%$


Example: $x=10$; the reverse operation of $+10\%$ is $-(100\cdot \frac{10}{100+10})\%\simeq -9.09\%$

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  • $\begingroup$ Thank you so much for your time commenting on my post! I'm still a bit confused, maths isn't my strong point I'm afraid. How would this work If I used as selling price of £1.56, added on 16.5% and then wanted to subtract 16.5% from £1.18174 to get back to £1.56? How would that work in that formula you supplied? Thanks in advance! $\endgroup$
    – AdamF
    Jul 26 '18 at 10:46
  • $\begingroup$ sorry I meant £1.8174 not £1.18174 $\endgroup$
    – AdamF
    Jul 26 '18 at 11:04
  • $\begingroup$ If you subtract the same percentage you don't get back to the initial amount. To get back to the initial amount after an increase by $x\%$, you need a decrease by $\frac{100x}{100+x}\%$. $\endgroup$ Jul 26 '18 at 16:28

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