Student: A certain car uses 12 gallons of gasoline in traveling 420 miles. In order for the car to travel the same distance using 10 gallons of gasoline, by how many miles per gallon must the car’s gas mileage be increased?\nOptions:\n(A) 2\n(B) 4\n(C) 6\n(D) 7\n(E) 10.
Another student: Let's say, hmmm...

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420/10 = 42 The difference is 42 - 35 = 7. Final answer: (D).
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