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)

Answer: d

解答为

28 A卖了X，B卖了Y

(1X+1.25Y)×r%=1X

(X+Y)×p%=X

两个联立得出400p / (500 – p)

但是我怎么不明白怎么两个式子出了一个这个结果呢...........

我的方法比较笨，就是通过第二个等式算出X=P%Y/1-P%，然后把它带到第一个等式里面去就好了。算的有点烦！

many thanks !

相对有一个简单的办法, 不用xy, 直接表达文字的意思是:

r%=1*P/(1*P+1.25*(100-P))

这样就可以直接得出答案了