１．Of the families in City X in 1994, 40 percent owned a personal computer. The number of families in City X owning a computer in 1998 was 30 percent greater than it was in 1994, and the total number of families in City X was 4 percent greater in 1998 than it was in 1994. What percent of the families in City X owned a personal computer in 1998?
A. 50%
B. 52%
C. 56%
D. 70%
E. 74%
key is A. why?
2.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)
key is D. why?
many thanks!!!
有题干可知，到1998年City X有电脑的为 40%A(1+30%)=52%A 户家庭；并且到1998年，City X有 (1+4%)A=104%A 户家庭。
由此可知，有电脑的家庭占总家庭数的比例为：52%A/104%A=50%
2. 总收入由TR表示，卖出报纸总数量由Q表示，卖出报纸A数量由A表示，卖出报纸B数量由B表示。
有题干可知，$1.00A+$1.25B=TR（即A+1.25B=TR），$1.00A=r%TR（即A=r%TR），A=p%Q；
由后两个等式可得出A=r%TR=p%Q，TR=(p/r)Q；由该商场只卖A&B可知B=(1-p%)Q；
代入等式A+1.25B=TR，可得p%Q+1.25*(1-p%)Q=(p/r)Q，即r=p/[p%+1.25*(1-p%)]=400p/(500-p)。
[该题应注意，由于题干一直在讲r percent & p percent，用等式表达时应注意表示成r% & p%~~]
谢谢！！！
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