First, you need to list the information that have been given.
A full tank of gas can hold <u>13 gallons of gasoline</u>.
Ryder used a full tank of gas to drive <u>273 miles</u>.
The question is asking for how many miles per gallon the car can travel on average. So, you have to divide the number of miles that a full gas tank can give by the amount of gallons a full gas tank is.
After dividing the values, you should end up with 21 miles per gallon.