A positive rational number is a number that can be expressed as a fraction where both the numerator and the denominator are positive integers.

To add two positive rational numbers, find a common denominator, convert the fractions if necessary, and then add the numerators while keeping the common denominator.

To subtract one positive rational number from another, find a common denominator, convert the fractions if necessary, and then subtract the numerators while keeping the common denominator.

The sum of 1/2 and 1/4 is 3/4.

The difference between 3/4 and 1/2 is 1/4.

Convert 2 1/4 to an improper fraction (9/4), convert 3/8 to a common denominator, and subtract: 9/4 - 3/8 = 1 7/8 gallons.

An improper fraction is a fraction where the numerator is greater than or equal to the denominator.