Finding total after percentage has been used? Tried my best with the title. Ok, earlier while I was on break from work (I have low level math, and want to be more fond of mathematics)
I work retail, and 20% of taxes are taken out, and I am wanting to find out how much I made before 20% is taken out.
So I did some scribbling, and this is without googling so please be gentle. but lets say I make 2000 dollars per paycheck, so to see how much money I get after taxes I do 2000 * .8 which gives me 1600. I can also do 2000 * .2 which gives me 400 and then do 2000 - 400 (imo I don't like this way since I have a extra unnecessary step)
Anyways, I was pondering how to reverse that to have the answer be 2000, and what I did was 1600 * 1.20 (120%) which gave me 1920, and I thought that is odd so I did 1600 * 1.25 and answer was 2000 exact.
My question is why did I have to add extra 5% (25%) to get my answer? I am sure I did something wrong, and I fairly confident the formula I am using is a big no no.
edit; wow thank you all for your detailed answers. I am starting to like mathematics more and more.

If $20\%$ of your pay is removed for taxes, then you retain $80\%$ of it.  Thus, the amount you receive after taxes is 
$$\text{net} = 80\%~\text{of gross} = \frac{80}{100}~\text{gross} = \frac{4}{5}~\text{gross}$$
To determine your gross pay from your net pay, you must multiply your net pay by $5/4$ since 
$$\text{gross} = \frac{5}{4} \cdot \frac{4}{5}~\text{gross} = \frac{5}{4}~\text{net}$$  
Since $$\frac{5}{4} = \dfrac{125}{100}$$ your gross pay is actually $125\%$ of your net pay.
