求解 一道数学

MoneyHunter

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)
答案D

Leliolevine

设 A卖出x1  B卖出x2    x1/(x1+x2)=p/100……1     x1/(x1+1.25*x2)=r/100……2    从1中得出x1/x2=p/(100-p)    从2中得出x1/x2=1.25*r/(100-r)     则p/(100-p)=1.25*r/(100-r)     r=400p/(500-p)