How many miles can you go with 42mpg?
Depends on how many gallons of gas your tank holds. If it holds 1 gallon, then you can go 42 miles. if it holds 2 gallons, you can go 84 miles.
What is the gallon-capacity of your car? In other words, how many gallons can do you need to fill your gas tank?
Multiply the gallon-capacity by 42 to get your answer.