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标题: PP2-PS-7 [打印本页]

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作者: Kath1985    时间: 2010-4-21 20:22
标题: PP2-PS-7
7.1502-!-item-!-187;#058&001086

A certain telephone company offers two plans, A and B.Under plan A, the company charges a total of $0.60 for the first 7 minutes of each call and $0.06 per minute thereafter.Under plan B, the company charges $0.08 per minute of each call.What is the duration of a call, in minutes, for which the company charges the same amount under plan A and under plan B ?



(A) 2

(B) 9

(C) 15

(D) 21

(E) 30

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【答案】B

求出來9 請問為什麼不用把7加回來 ?  
For A: Charges=0.6+0.06(t-7)
For B: Charges=0.08t
依照題意charges相同è0.6+0.06(t-7)=0.08t
è0.02t=0.6-0.42=0.18
èt=0.18/0.02=9min

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作者: 草稿纸    时间: 2010-4-21 20:25
你设的是总时间啊，所以不用加7

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作者: Kath1985    时间: 2010-4-21 23:09
哈哈 剛剛也想到  謝謝你 心有靈犀

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