A store receives a shipment of 1000 greeting cards. Each day, the store sells 25% of its stock of cards. After 15 days, how many cards will remain in the store?

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?