To determine which store has the better buy for a gallon of gasoline, we need to compare the cost per gallon for both stores.
In Store A, the cost of gasoline per gallon increases by $20 for every 5 gallons.
In Store B, the cost of gasoline per gallon increases by $18.75 for every 5 gallons.
Since the cost per gallon in Store A increases by a larger amount than in Store B, this means that Store B has the better buy for a gallon of gasoline.
Therefore, the answer is 2 for Store B.