Question

Linda commutes a total of 54 miles to and from work every day. She needs a new car, and good gas mileage is important to her. She’s considering two models. The first model averages 30 miles per gallon, and the second averages 27 miles per gallon. How many gallons of gas will she save in a five-day work week if she buys the first car instead of the second car?

Answer 1

The answer is 1 gallon.

Miles per gallon(mpg) is computed by dividing the distance traveled by the how many gallons used. So you can derive a formula for how many gallons you would use given the mpg. You will end up with:

$gallon = \frac{miles travelled}{miles per gallon}$

The problem asks for how many gallons of gas she will safe in a five-day work work week. So first you need to compute how many miles that would be. 

54 miles/day x 5days =  270 miles

So in a five day work week, she will travel 270 miles.

Now to see how much gas she will save, compute how many gallons she will use up for each car, given the mpg of each and find the difference. 

First model:30 mpg

$gallons = \frac{270miles}{30mpg}$
$gallons = 9g$
 
This means that with the first model, she will have used up 9 gallons in a 5-day work week.

Second model: 27 mpg
$g = \frac{270mi}{27mi/g}$
$g = 10g$

This means that with the second model, she will have used up 10g in a 5-day work week. 

Now for the last bit. How much will she save? You can get that by getting the difference of how many gallons each car would have used up.

10gallons - 9gallons = 1g

So she would have saved 1 gallon of gas if she buys the first car instead of the second. 
 

Answer 2

In a five-day work week Linda commutes a total of 270 miles ($5\cdot 54=270$ miles).

If she buys the first car, she will spend $270:30=9$ gallons of gas and if she buys second car, she will spend $270:27=10$ gallons of gas.
So, she will save 1 gallon of gas in a five-day week
if she buys the first car instead of the second car