The case mentions 2 different price elasticities: one for the adult population as a whole, and the other for a subset of the population: young adults between the ages of 15 and 24 years old. Plese give a brief explanation og what these actually mean i.e. explan these two measures as if you were explaining them to an intelligent person, who had no knowledge of the concept of price elasticity of demand.

Can you suggest some reasons these two groups may have different price elasticities of demand??

See my answer to another question