Problem Solving

BTGmoderatorDC wrote: ↑

Fri Nov 12, 2021 7:37 pm

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



OA B

Source: GMAT Prep

In other words, we want to find the value of x such that: (total payments WITH sticker) < (total payments WITHOUT sticker)

total payments WITHOUT sticker
If the driver crosses the bridge twice on x days, but then the total number of crossings = 2x
Since each crossing will cost $0.75, the total payments for the month = (2x)($0.75) = 1.5x

Total payments WITH sticker
The sticker costs $13.00
If the driver crosses the bridge twice on x days, but then the total number of crossings = 2x
Since each crossing will cost $0.30, the total payments for the month = $13.00 + (2x)($0.30) = 13.00 + 0.6x


So, our inequality becomes: 13.00 + 0.6x < 1.5x
Subtract 0.6x from both sides: 13 < 0.9x
Divide both sides by 0.9 to get approximately: 14.44 < x

Since x must be a positive integer, the smallest value of x is 15

Answer: B