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