> Is it cheaper to buy 1 case of water for $2.50 or 3 cases of water for $9.00

Slightly off topic as I don't think this is college level algebra, in supermarkets in Europe they are required by law to put the "€/liter" price in small on the side, so you can compare easily. In the US it doesn't seem mandatory, or at least the unit is not mandatory because sometimes they use fl oz, sometimes gallons, or for pieces of meat sometimes oz, sometimes lbs, so when you want to compare the prices of 2 similar items, what would just take you 0.5 sec in Europe takes you much longer in the US since you need to add unit conversion in the mix. For example here your "cases" could be of different sizes, that 1 case of water could be 2L ,while those 3 cases could be a gallon each...