procento

Here is the seam that looks real and is not. Esperanto has a preposition po meaning at the rate of, and a numeral cent, and procento sits there looking as though somebody welded them together. It was not built that way. Procento is one borrowed root, and the pro- at the front is not Esperanto's pro either.

What makes it a good trap is that the Latin genuinely is that build: per centum, by the hundred. So the morphology you can see in the word is real history and false grammar at the same time. That combination turns up again and again in the international roots. Inspiri looks like in- plus something and its Latin ancestor really was in plus spirare, to breathe in, but Esperanto took the whole thing as one piece. You cannot swap the prefix and get a new word.

For actual arithmetic, dek procentoj de io is ten per cent of something, procenta is the adjective, and procentaĵo the percentage as a figure. Bank interest is a procento too, or interezo if you want to be precise about it.