[econ]點解係餐廳賣o既可樂會貴過超市咁多?

Price elasticity of demand. When you are consuming food in restaurant, your elasticity of demand on soft drinks is smaller than one ( inelastic demand), you can not go to other places to buy drinks and take the drinks back to the restaurant. However, in Park'N shop, you have abundance of choices e.g. you have time to go to other supermarkets for cheaper coca cola. Thus, the demand is more elastic.
You can draw a demand curve with steep slope ( inelastic demand) even pushing up the price significantly, the total expenditure is still increasing.