Final answer:
To find the miles per gallon, divide the number of miles driven by the number of gallons used.
Step-by-step explanation:
To find the miles per gallon, we need to divide the number of miles driven by the number of gallons used.
Let's say the number of miles driven is x. Since the tank is full when he left, and it took 13 gallons to refill, we can assume that he used x miles of gas.
Therefore, the miles per gallon is x/13.