You are visiting your cousin who lives in the city. To get back home, you take a
taxi. The taxi charges $2.10 for the first mile and $0.90 for each additional mile.
After riding 13 miles, you decide that the fare is going to be more than the $20
you have with you. So, you tell the driver to stop and let you out. Then you call
a friend and ask your friend to come pick you up. After paying the driver, how
much of your $20 is left?