Seth bought 18.4 gallons of gasoline for $46.74. What was the unit price of the gasoline? A. $3.96/gallon B. $2.59/gallon C. $2.54/gallon D. $4.08/gallon

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.