This question requires an important assumption to be made. This being that the dose varies linearly with age. As the age of the adult is not provided it is taken as A.
The dosage of aspirin for an adult that is A years old is 325 mg. For a child of age 8, the dose is x where 8 : x = A : 325
=> x = 325*8/A
=> x = 2600/A
The amount of aspirin to be given to a child of age 8 is 2600/A where A is the age of the adult and the adult dosage is 325 mg.