大疑惑！！！求助！

gzzxl · 发表于 2013-11-15 08:59:27

During a sale, a clothing store sold each shirt at a price of $15 and each sweater at a price of $25.
Did the storeell more sweaters than shirts during the sale?
(1) The average (arithmetic mean) of the prices of all of the shirts and sweaters that the store sold during the sale was $21.
(2) The total of the prices of all of the shirts and sweaters that the store sold during the sale was $420.
题目翻译:15 刀一件 shirt,25 刀一件 sweater。问卖的 sweater 是否比 shirts 多思路:A—Shirts, B—Sweater, B>A?
仅条件一:15A+25B=21(A+B) 推出 3A=2B 故 B>A, 充分仅条件二:15A+25B=420, 不充分
选A
这个题第二个不充分推不出来，可我记得曾经有一个人就是问类似这样的15A+25B=420方程有几个解！！那时候就只有一个解！为什么这次不行。。。？不知道大神们有没有明白我的意思。。。

zdwangyouqi · 发表于 2013-11-15 09:17:49

B肯定推不出来啊，shirt 13,sweater 9就是反例。还有，15A+25B=420 中B肯定是3的倍数，试几个就有反例了。

大疑惑！！！求助！

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