VAT Percentages calculator

I made a simple VAT calculator that when you put a price in without VAT it calculates the 20% TAX on it and then if you put a price with the VAT on it it will take it off to show you the true cost of the item.

So when I input £100 it shows that with VAT its £120. I then for example place the £120 price into the next setting which removes the VAT from the price, 20% of £120 is £24 not £20... how? How is £100 at 20% £120 but then £120 without the 20% £96? I feel like i'm brain damaged or something, jesus. I just cant figure it out!!