Answer:
Miles: 35; Gallons: 1
Explanation:
The number of gallons can be any number of your choosing that is less than 12, which is the capacity of the tank. For the purpose here, 1 is a convenient number. To find the corresponding number of miles (m), you need to solve the proportion ...
420/12 = m/1
35 = m
(miles, gallons) = (35, 1) . . . a suitable choice for the table values
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Additional comment
If your sense of symmetry is offended by choosing 1 for Gallons, you can choose 10, a number halfway between 12 and 8. In that case, your proportion is ...
420/12 = m/10 ⇒ m = 350
Then, your entry would be ...
(miles, gallons) = (350, 10)