Dolly has decided to earn a little extra money over the winter by shoveling snow.  She determines that her total...

Dolly has decided to earn a little extra money over the winter by shoveling snow.  She determines that her total cost of shoveling is $7.50 per driveway regardless of how many driveways she does.  One neighbor is willing to pay

Dolly $15 to shovel.  A second neighbor is willing to pay $10. A third is willing to pay $5 and a fourth is willing

to pay $4. How many driveways should she shovel?