Tina must make a decision about the number of bottles of sports drink and how many cases of water she wants to purchase for her soccer practices. Tina decided to buy four cases of water and 16 bottles of sports drink. As she waited in line to check out, Tina noticed there was a special display offering five cases of water for the price of four. Tina opted to pick up an additional case of water, even though she might not use it until next week. Tina exhibited the economic principle of , because she decided that five cases of water and 16 bottles of sports drink was better than four cases of water and 16 bottles of sports drink.