Rational number word problem: cab

Sandy is taking a cab in New York City. The taxi fare includes a base charge of $2.50 as soon as you enter the cab, plus an additional $2.50 per mile traveled. If Sandy starts off with a $20 bill, takes the cab for five miles, and leaves a $1 tip, how much money will she be left with? Well let's first just think about how much this five mile cab ride is going to cost her. So cab-- I'll call it the cab fare. So it's going to be $2.50 just when you get into the cab, plus $2.50 per mile. Well she's going to go for 5 miles-- let me write it this way-- times $2.50 per miles. So the cab fare here is just going to be $2.50, this $2.50, plus 5 times $2.50. Let's see, 5 times $2.50, that is $12.50. So this is plus $12.50. $2.50 plus $12.50 is going to be $15. So just the cab fare is going to be $15. And then Sandy gives a $1 tip. So fare plus tip is going to be equal to the cab fare $15 plus $1 which is equal to $16. So then answering the question, how much money would she be left with with a $20 bill? Well we could write her change is going to be, she's going to give a $20 bill, the fare plus tip is $16, minus $16. So she's going to have $4 left in change.