Today at work I was trying to understand whether a given amount (revenue) is with or without VAT. (vat is 21%).

Here is the amount with VAT (1980.50)

Here is the amount without VAT (1636.4)

The way I've taught myself to add or subtract a percentage from an amount is to either multiply by 1.21 to add 21% or to multiply by 0.79 to subtract 21%.

Here is what confused me today.

If i do this calculation to add 21% to 1636.4 (1636.4 * 1.21), i get 1980.044 (this is correct)

If i do this calculation to subtract 21% from 1980.5 (1980.5 * 0.79) I get 1564.6 (expected to 1636.4)

Why am I not getting 1636.4 when i subtract 21% from 1980.5?

Also. i was told by my colleague to subtract 21% from 1980.5 I am supposed to divide by 1.21. This way the

calculation makes sense (1980.5 / 1.21 = 1636.77).

Why my colleague's way works and why mine does not?

Thanks in advance for your help!