求教一道数学题

viviangao1110

a pharmaceutical company received 3 million in royalties on the first 20 million in sales of the generic equivalent of one of its products and then 9 million in royalties  on the next 108 million in sales. By approximately, what percent did the ratio of royalties to sales decrease from the first 20 million in sales to the next 108 million is sales?
A 8%
B 15%
C 45%
D 52%
E 56%
答案是c
解释：（9/108-3/20)/(3/20)=-0.44
           所以近似为45%
不明白为什么减法之后还有除以108million的那个比率呢？