Q28:

Last Sunday a certain store sold copies of Newspaper A for $1.00 each and copies of Newspaper B for $1.25 each, and the store sold no other newspapers that day. If r percent of the store’s revenues from newspaper sales was from Newspaper A and if p percent of the newspapers that the store sold were copies of newspaper A, which of the following expresses r in terms of p?

A. 100p / (125 – p)

B. 150p / (250 – p)

C. 300p / (375 – p)

D. 400p / (500 – p)

E. 500p / (625 – p)

Key is D, 不知道怎么算出来的。。。

x=revenue, y=sales;

(1) x*(r/100)=y*(p/100)*1

(2) x*(1-r/100)=y*(1-p/100)*1.25

(1)/(2)----> D

A/(A+B)=P/100

A/(A+1.25B)=r/100

其中A,B是卖掉的报纸份数(copies)