(1 gallon)/(100 ft^2) is a good conversion factor. We could also use (100 ft^2)/(1 gallon), but I like the first, since the calculation is simpler:
(300ft^2)*/(1 gallon)/(100 ft^2) = 3 gallons [I like multiplication better than division, and especially here, since 300/100 is 3, if my memory is correct.
