AMC8 2007

AMC8 2007 · Q6

AMC8 2007 · Q6. It mainly tests Percent, Averages (mean).

The average cost of a long-distance call in the USA in 1985 was 41 cents per minute, and the average cost of a long-distance call in the USA in 2005 was 7 cents per minute. Find the approximate percent decrease in the cost per minute of a long-distance call.

1985年美国长途电话的平均费用是每分钟41美分，2005年是每分钟7美分。求长途电话每分钟费用的大致百分比减少量。

(A) 7 7

(B) 17 17

(D) 41 41

(E) 80 80

Answer

Correct choice: (E)

正确答案：（E）

Solution

(E) The difference in the cost of a long-distance call per minute from 1985 to 2005 was $41-7=34$ cents. The percent decrease is $100\times\frac{34}{41}\approx 100\times\frac{32}{40}=100\times\frac{8}{10}=80\%$.

（E）1985 年到 2005 年长途电话每分钟费用的差额为 $41-7=34$ 美分。降幅百分比为 $100\times\frac{34}{41}\approx 100\times\frac{32}{40}=100\times\frac{8}{10}=80\%$。

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