[求助]请教NN们这种类型的数学题

这种题我总是不知道怎么设定未知数来列方程，请大家不要笑我，每次遇到这种题就犯傻。

能不能给我讲一下这种题的解题思路，非常感谢！！

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)

D:400p / (500 – p)

这种题一般设总卖出的newpaper量是a，总revenue为b

(1)p/100*a+1.25a*(100-p)/100=b

(2)p/100*a=r/100b

从(2)得b=pa/r，带入（1）消去a只剩r，p。用p表示出r即可

这类题型，一般就是设总量，每一部分可以用它占总数的百分比表示。

然后根据题意，把所有的关系式列出，可以消去一些没用的。