快考了 数学题 帮帮忙

jojo1130zy

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?

答案是 400p / (500 -  p )

这题我不会 谁会啊 帮帮忙 谢啦

usocean

题目没看明白？哪位给解释一下吧！

yoles

设卖出了a份A,b份B. a*1/a+1.25b=r/100   a/a+b=p/100   带入算即可。

yoles

r%收入来自于A报纸，p%的销量也来自A报纸

hu1ha2rachel

以下是引用yoles在2009-6-4 21:49:00的发言：

设卖出了a份A,b份B. a*1/a+1.25b=r/100   a/a+b=p/100   带入算即可。

两式相比就可以得出a/b，然后带入任意式，计算即可~

海水正蓝

我怎么觉得答案错了

weiwei_zhwq

设总共卖了100分, A报卖了P份,  B报卖了(100-P)份

总收入= 1*P+1.25*(100-P)

R%=1*P/1*P+1.25*(100-P)=P/P+125-1.25P=P/125-0.25P

R=100P/(125-0.25P)=400P/500-P

weiwei_zhwq

以下是引用hu1ha2rachel在2009-6-4 22:00:00的发言：

两式相比就可以得出a/b，然后带入任意式，计算即可~

麻烦能把具体得步骤写一下吗?...两个式子我能理解, 但是怎么个带入法可以用P来表示R呢? 拜托!

usocean

看来我是题目没有读懂。r表示A在销售金额中的百分比，p表示A在份数中的百分比。谢谢。