Prep T2-PS-7

[w062611]

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

答案為 B

題目就看不太懂了

可以請高手幫忙解一下嗎

感謝了

[scottkidd]

A公司的費率：前七分鐘共0.6元，之後每分鐘是0.06
B公司的費率：每分鐘0.08元
要求兩間公司計費相同的時間，所以是
0.6+0.06(x-7)=0.08x
這樣應該就算出來了

[chris8888]

假設打了x分鐘,
0.6(前七分鐘) + (x-7)*0.06 = 0.08 * x
0.18 = 0.02x
x = 9