One way to make this easier is to write the units.
You have $16.80 / 4 gallons. That's $4.20 per gallon, or we can write 4.20 dollars/gallon.
Since you want gallons per dollar, just invert the 4.20:
N gallons / dollar = (1/N) dollars / gallon:
1/ (4.20 dollars/gallon) = 0.2381 gallons / dollar.
I hope this makes sense.
(Sanity) Check: 0.2381 gallons for $1 … That's a little less than one quarter gallon for $1, so if I wanted an entire gallon it would be a little over $4. Looks reasonable.
