請教TTGWD15-29

suibian

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 $13.00 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

這邊不是很懂

請指教．．．謝謝

plmnpler

use sticker: 0.3*(2x)+13

without using sticker: 0.75*(2x)

in order to save money with sticker: 0.3*(2x)+13-0.75*(2x)<0

Therefore: x>(13/0.9) approximately 14.444

Thus, the ans=15