[求助]天山 4-29

poorleafs

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

kuuiytt

我的解法是:

(31/x)(2)(0.3)+13 < (31/x)(2)(0.75)

求X的最小值....

可是怎么算不出來啊...

wretdgdh

X如果是相隔的天数的话，最小值应该是1的，我想它应该问的是x的最大值，就是15没错.

girls

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