Q29
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 and then pay only $0.3 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
The answer is B but I think the answer is A
0.75×2 ×N<13+0.3×2 ×N
我的解法是:
(31/x)(2)(0.3)+13 < (31/x)(2)(0.75)
求X的最小值....
可是怎麼算不出來啊... 邏輯哪裡錯了呢?
发表一下愚见
cross the bridge twice on each of x days next month : 在下月,X天中的每一天过桥两次,即为只有X天过桥,因此过桥总次数为 2X,
使用sticker省钱,则其成本必然小于等于没有使用sticker的成本
13+2X * 0.3<=2X * 0.75 解之得:X>=14.4 故最小值为15 答案B
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