We were told that sam drank 1/2 of a bottle of soda and duncan drank 1/6 of a bottle. Thus, the quantity that Sam drank more than Duncan is
1/2 - 1/6
= The lowest common multiple is 6. It becomes the common denominator. We would multiply each denominator by 6 and multiply by the numerator. Thus, we have
(3 - 1)/6 = 2/6 = 1/3
Thus, Sam drank 1/3 more soda than Duncan
