Why is "de" used here instead of "da"?
Because "dek" is part of "cent". (Note: I am by no means fluent in Esperanto)
I think it's because it's a definite quantity?
10 percent = 1 tenth
Couldn't decide if it was "Ten is a tenth of one hundred" or "Ten is one tenth of a hundred". I guess I guessed right.
Update: it works either way