The difference between two numbers is -1 and their sum is 1. Find the two numbers. A: 0 and 1
B: 1 and -1
C: 2 and 3
D: 3 and 2

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?