Chemistry: The Central Science (13th Edition)

$(\frac{1.89 dollars}{gallon})\times(\frac{1 gallon}{3.78541 Liters})=.50 dollars/liter$ In this problem, it is necessary to multiply the given number by $(\frac{1 gallon}{3.78541 Liters})$. First, by putting gallons in the numerator, it cancels out the unit gallons, for this unit is in the denominator of the previous fraction. This leaves the desired unit, dollars per liter, left over. Secondly, note that one gallon equals 3.78541 Liters. Thus, when multiplying by $(\frac{1 gallon}{3.78541 Liters})$, the numerator equals the denominator. This fact makes this step legal, for if the numerator and denominator are not equal, it would be like multiplying by a number not equal to one, which is illegal in unit conversions. Finally, because there is no half cent, the answer is rounded to the nearest whole cent.