since the dragster can travel 1/4 of a mile in just 4½ seconds, how long will it take it to travel a whole 1 mile?
well, a whole mile is 4/4 of a mile, 4/4 = 1, since 1/4 of a mile takes 4½ seconds, 4 times that much is just 4½+ 4½+ 4½+ 4½, namely 18 seconds.
so it does a whole mile in 18 seconds, and on each of those 4½ intervals, it burned 6gallons of fuel, namely 4 * 6 gallons.