If 1500 is deposited in an account that pays 4% interest, what is the difference in the amount after 3 years between the amount earned if the principal is compounded annually and the account earned calculated using simple interest

Last month, a 770 g box of cereal was sold at a grocery store for $3.10. however, this month, the cereal manufacturer launched the same cereal in a 640 g box, which is being sold at $2.50. what is the percent change in the unit price?