Comparing all the combinations, the best buy is the "six 5-gallon jugs at 100 each + two 1-gallon jugs at 15 each", which costs:
(6 x 100) + (2 x 15) = 630 (32 gallons)
The costs of the other combinations are:
542 + 208 = 750 (35 gallons)
542 + 100 + (2 x 15) = 672 (32 gallons)
(3 x 208) + (2 x 15) = 654 (32 gallons)