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November 17th, 2018, 08:10 AM  #1 
Newbie Joined: Nov 2018 From: UK Posts: 1 Thanks: 0  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 = Â£350.00 MaxBid18 = Â£413.00 My formula =(MaxBid + (18/100*MaxBid) WithVAT20 = Â£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 = Â£495.60 Minus20 = Â£495.60  (20/100*Â£495.60) = Â£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 10:52 AM. 
November 17th, 2018, 11:07 AM  #2 
Global Moderator Joined: Dec 2006 Posts: 20,648 Thanks: 2085 
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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