Bava Metzia 54 - Adding One-Fifth

When one redeems the second tithe taken from his produce for money, he needs to add 1/5. There are many other cases where 1/5 is added, such as redeeming property belonging to the Temple treasury or reapying a Kohen’s portion (trumah) that one has accidentally eaten. Is the 1/5 calculated on the principal ("from the inside) or on the principal plus the addition ("from the outside")?
For example, adding a 1/5 to 20 can mean paying 4 (1/5 of 20), or it can mean 5, since the total then is 25, and 1/5 of it is 5.
The answer is that 1/5 is calculated from the outside, so it really is 1/4 of the principal.