Henry bought a car for $14,000. After two years, the value of the car was $8,400. What was the percent decrease in the value of the car after two years?

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?