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Hi I have an item for £4000 which should have a reduction of 20% over the year.

I want to calculate the value for each month. What equation would I use?

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If the reduction should be a constant number of pounds per month, you would use $4000(1-\frac{0.2*months}{12})$

If the reduction should be a constant fraction per month, the value would be multiplied by $\sqrt[12]{0.8}$ each month, so the value would be $4000*0.8^{\frac{months}{12}}$

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