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November 17th, 2018, 09:10 AM   #1
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Calculating percentages

Hi, I have an issue calculating these percentages.

I have a spreadsheet that applies a rate of 18 % to a figure and then an additional 20 % to the total.

I am trying to create a spreadsheet that will take a total figure and miss 20 % then an additional 18 % to give me my initial figure but the calculation is different.

Here's an example:

MaxBid = £350.00
MaxBid18 = £413.00 My formula =(MaxBid + (18/100*MaxBid)
WithVAT20 = £495.60 My formula =(MaxBid18 + (20/100*Max Bid18)

So far this seems correct to me...

Now I try to reverse engineer this calculation to give me a figure to a MaxBid:

WithVAT20 = £495.60
Minus20 = £495.60 - (20/100*£495.60) = £396.48
Minus18 = ....

However I can see I not getting the same result as before.

Can someone please correct me?

Thanks

Keith

Last edited by skipjack; November 17th, 2018 at 11:52 AM.
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November 17th, 2018, 12:07 PM   #2
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MaxBid + (18/100*MaxBid) = MaxBid * (1 + 0.1 = MaxBid * 1.18, so the calculation to reverse engineer this is MaxBid = MaxBid18 / 1.18, and (similarly) the calculation to reverse engineer the VAT should be MaxBid18 = WithVAT20 / 1.2.

Note that adding a percentage to a price might also involve rounding the result, and then reversing the calculation might not give precisely the original amount.
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