Eight ounces (which is abbreviated as eight oz) is also equivalent to one cup (which is abbreviated as one c), two cups (which is abbreviated as two c) is equivalent to one pint (which is abbreviated as one pt), two pints (which is abbreviated as two pts) is equivalent to one quart (which is abbreviated as one qt) and four quarts (which is abbreviated as four qts) is equivalent to one gallon (which is abbreviated as one gal).
Therefore the calculation which we use to convert cups to gallons is taken as x (the unknown cups, which is denoted in algebra by the variable x) divided by four quarts (which is one gallon) is equivalent to four cups divided by one quart. If the denominator, which is one quart, is multiplied by four, it gives four quarts, which is equivalent to one gallon.
By the same algebraic solution, if the numerator, which is four cups, is multiplied by four, it gives 16 cups, which is equivalent to one gallon. Therefore one gallon is roughly equivalent to sixteen cups.
