Chinese percentages are formed as 百分之 + number — literally 'parts of 100, of which N.' So 百分之十 = 10%, 百分之五十 = 50%. The 之 (zhī) is the classical possessive 'of.' Same pattern works for any 'X out of Y': 三分之一 (sān fēn zhī yī) = 1/3.
mistakes
English speakers often want to say 二百分 thinking 'two of 100' — but Chinese flips it: the WHOLE comes first, then the part. 百分之二 = 'of-100, two,' i.e. 2/100.
