Multiply by conversion factors that cancel the units you don't want and leave the units you do want. Each factor should be equivalent to 1. That is, the numerator and denominator should have equal value. For example, 26 miles per gallon means 1 gallon is equivalent to 26 miles.
[tex]450\,\text{mi}\times\dfrac{1\,\text{gal}}{26\,\text{mi}}\times\dfrac{\$3.30}{1\,\text{gal}}=\dfrac{\$3.30\cdot 4.50}{26}\approx \$57.12[/tex]
