prep 不同收费计划问题

songxuebest · 发表于 2010-8-6 16:11:35

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

【思路】設t為時間

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

就是最后这一步：为什么除以0、02呢

就是最后这一步：为什么除以0、02呢

songxuebest · 发表于 2010-8-6 17:04:07

哭泣

songxuebest · 发表于 2010-8-6 17:24:44

哭泣没人管

vague0909 · 发表于 2010-8-6 17:30:18

6+0.06(t-7)=0.08t
6+0.06t-0.42=0.08t
0.18=0.02t
t=9

prep 不同收费计划问题

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