Answer:
You didn't show the problem so ill just make it to where you can answer yourself
Explanation:
so if he uses 3 gallons per 21 miles just divide 3 by 21(yes 3 by 21)
So 0.142 is used per mile
So because you don't show how many miles he traveled just take the number of miles he traveled and times it by 0.142
Example: 42 x 0.142 = 5.964 which when rounded is 6