1. You have that Chris used 18 gallons of gas in 420 miles.
2. Therefore: If Chris drove 420 miles using 18 gallons of gas, how many gallons would in 357 miles?
3. Let's call "x" to the amount of gallons he need to drive 357 miles. So, you have:
420 miles----18 gallons
357 miles-----x
x=(357)(18)/420
x=6426/420
x=15.3 gallons