GWD18-23

[jerry7271]

查尋過了沒這題
The toll for crossing a certain bridge is $0.75 each crossing. Drivers who frequently use the bridge may instead purchase a sticker each month for $13.00 and then pay only $0.30 each crossing during that month. If a particular driver will cross the bridge twice on each of x days next month and will not cross the bridge on any other day, what is the least value of x for which this driver can save money by using the sticker?
A.14
B.15
C.16
D.28
E.29
答案是B。 但我是算A，想法如下
以一個月30天計
(1) 30天x2次x0.75元=45
(2) 13元+0.3元 x2次x30天=31
所以45-31=14

不知道那裡算錯，請指教

[pri]

一個月有X天要過橋
若沒有買月票: 一天兩次,一個月共0.75*2*X=1.5*X
若有買: 13+0.3*2*X=13+0.6X
13+0.6X < 1.5X
13 < 0.9*X

=> X 最少要15天