Which way of calculating VAT is the correct one and why

Question

I kept searching for the logic of the vat calculations, but i still can't understand a few things,maybe you can help me out.

If i want to extract the price without tva,i'll be using this formula:

1. PriceWithoutTva = PriceWithTva / 1.19(for a tva rate of 19%),which i can't understand because for something like this: 100/1.19 = 84.02521008403361 , and that makes the TVA value to be almost 16 , but 19% of 100 is clearly 19.

If i want to extract the TVA from a price with tva,some say to use the above formula and simply decrease it from the PriceWithTva,which makes sense. Other says to multiply with the VAT percentage like this:

2. PriceWithTva * 0.19(for a tva rate of 19%) ,which makes sense because if i have 100 *0.19 it will give 19 which it's logical.

But then,i came across a formula which is widely used if i want to extract the TVA from a PriceWithTva:

3. PriceWithoutTVA = 100/(100+19) * PriceWithTVA , and the result is 84.02521008403361 which makes the TVA value almost 16 again for a PriceWithTVA of 100

Another formula which i can't understand is:

4.TVA = PriceWithoutTVA * 0.19(tva rate) , which again gives almost 16 for the PriceWithoutTVA of 84.02521008403361

Thanks for help.