"The fraction one-fifth is likewise common (Lev. 5:16; 22:14)." "In ritual observances the fraction one-tenth occurs frequently (Num. 28)." "The term pi shenayim originally meant two-thirds but subsequently came to signify "twice as much" (II Kings 2:9)."
As for the translation, I used NIV. Some others will actually say 20% for that quote.
My interest was piqued. I've asked a question at one of the Stack Exchange sites.
(http://hermeneutics.stackexchange.com/questions/2885/do-the-...)