You can find the answer by subtracting the amount of water he used by the amount of sugar he used. (1 5/6 - 3/8 = 1 11/24) To subtract mixed fractions, find a lowest common denominator by multiplying both of the denominators with different numbers until both denominators are the same. Then simply subtract. For example, multiplying 6 by 4 and 8 by 3 both get 24, so you can multiply the numerators by the same integers (5 by 4 and 3 by 3) to get the numerators on an equal fraction.