Last Sunday a certain store sold copies of Newspaper A for $1.00 each and copies of Newspaper B for $1.25 each, and the store sold no other newspapers that day. If r percent of the store’s revenues from newspaper sales was from Newspaper A and if p percent of the newspapers that the store sold were copies of newspaper A, which of the following expresses r in terms of p?
~$\frac{100p}{125-p}$~
~$\frac{150p}{250-p}$~
~$\frac{300p}{375-p}$~
~$\frac{400p}{500-p}$~
~$\frac{500p}{625-p}$~
题设来自A 的份数比例是P,那么来自B的份数比例就是100-P
A和B的价格比是1/1.25=4/5,若A是4P,B的收入就是500-5P
然后,A占所有收入的百分比是r,所以4P/(500-5P+4P)=r/100所以r=400P/(500-P)
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