The merchant sold x gallons to one customer and 2x gallons to the other. That means he sold 3x (x + 2x) gallons of wine. So the five barrels of wine must add up to a number of gallons that is evenly divisible by three.

If the beer is:	The wine is:
      15                104
      16                103
      18                101
      19                100
      20                 99
      31                 88

Of the six values that could represent the quantity of wine sold, only 99 is divisible by three. Therefore, the beer is in the 20 gallon barrel. One customer bought the 15 and the 18 gallon barrels (33 gallons). The other customer bought 66 gallons (16 + 19 + 31).